Ethics of staying in PhD program with no intention of being an academic, and misleading my advisor about this

As a PhD student, you don't have to want an academic career. It's not unethical to pursue a PhD with no intention of staying in academia afterward.

But lying about your plans and deliberately misleading is wrong and unfair to your advisor.

You wrote:

However, my advisor seems to be especially invested in making me a viable academic researcher because I am their first student and have used a lot of their startup funds. My field is also not typically industry-bound like, for example, engineering, so any reputation boost my advisor could hope to get from me is nil.

If your advisor is a new professor whose own career success really depends on graduating PhD students who go on to academic careers, letting them invest everything in you under false pretenses is obviously wrong and unethical.

It sounds like you don't really want to finish your PhD (I infer this from your "apathy for my subfield and academia in general"), and you think your advisor wouldn't want you to stick around for a few more years under the circumstances, either. On the other hand, finishing out the academic year - giving you time to look for a job, and giving your advisor time to find another PhD student in this application cycle, while you wrap up and publish your current work - sounds like a win-win. But you should tell your advisor right away, so that the two of you can make informed plans for the future.

I suggest you come up with a plan for preparing yourself for the job market during this academic year, then have that talk with your advisor, and make the most of your remaining time.

On the other hand, if you really do want to finish your PhD, tell your advisor that you're not interested in academia, but are interested in finishing your PhD and getting a job that takes advantage of your PhD-level research skills. (Many of the skills acquired by successful PhD students are fairly transferable.) Your advisor may be more willing to support you than you think - successfully graduating PhD students is important for new professors, even if the student isn't interested in academia in the end. But you need to let your advisor make that choice: lying to someone so that they will give you multiple years of research support just so that you can buy time to train yourself for an unrelated career, is wrong.

Questions of ethics are always murky. Let's try to untangle it a bit.

Staying in PhD program with no intention of being an academic.

There is no ethical dilemma in this. Really. I don't know where the notion came from that PhD studies should only develop future professors. It does not and cannot work like that, otherwise every professor could, on average, only supervise a single student his entire career, or professors would proliferate exponentially.

Misleading my advisor about this.

Misleading your advisor is quite obviously a problem. The question is why you need to mislead your advisor in the first place. As said above, it should be completely ok to work hard on your PhD and go to industry afterwards.

to stick around and beef these up until I see an opportunity to leave.

Ok, now we have a big problem. You are essentially intending to misuse your funding to not work on your research, but to ramp up your industry career. That's not ok and you probably know it. Contrary to your statement, this is also decidedly not the same as trying for an academic career and not making it. It's the same as getting a grant / stipend to do research and then using it for unrelated personal training.

Let me make this clear: you are paid (in salary or stipend) to do research, not to learn to program and build a network for industry job searching. Pretending to do the one and then doing the other is obviously unethical, even if you would not need to lie to your advisor about this. There may be some synergies (e.g., you build your network through research interactions with industry, or you learn to program because your research requires it), but doing things entirely unrelated to your research because you suspect they help you find a job is not ethical. It only gets worse if you need to lie about doing it.

I am willing to pick up the pace with my research and get the PhD.

Great! Do that instead. Do not misuse your time to train for an industry job, but finish your PhD. Conduct the best research you can. Collaborate and write papers. See if you can twist your research in a way that you learn more skills that will help in industry (not do things unrelated to research, but try to find synergies if possible). Train for a potential industry career on the side, outside of work.

Tell your advisor exactly this. You are not sure if you would want to stay in academia, but you will be writing the best dissertation that you possibly can. Don't get all emotional about it, or talk about apathy for your field. Don't make it your or your advisor's fault either - you just found over the long time of a PhD that academia, while interesting, is ultimately not for you. It happens to many (most?) students. Your advisor may be disappointed, but unless there is already a big conflict between the two of you, I cannot imagine him kicking you out over this.

I think the big misunderstanding here may be that you think your main contribution to your advisor's CV is you becoming a professor yourself, and otherwise you are worthless to him. Without knowing your university, your advisor's promotion case, or any other details, I am still willing to bet that this is not the case.

The main career benefit to your advisor lies in the research results you produce, and the papers you write. A secondary factor may be that you graduate, independently of what you do after. That is, in many universities, it is important for tenure and promotion cases that you have a certain number of students that graduated successfully. It always looks nice to be able to say that your students have been successful afterwards (for some definition of "success"), but I really think that the other two factors are much more important to your advisor. Just make clear that your decision to go to industry does not mean that you will try to do the weakest PhD that they let you graduate with (and mean it), and you should be fine.

I am a PhD since many years now. I have been in and out of academia since 2005. I was just like you.

I think that you really should consider completing your PhD programme. The reason for this is that you cannot judge, yourself, whether your progress is going well or not. Some students start really slow, but get some insights or discoveries later that are really impressive. I know that "others" performed more than me during their first years, but in the long run there is another discussion. Science is a slow process, and we cannot tell beforehand if a graduate student will become a good or successful researcher. I was also shocked how different professors thought of us grad students. Some where really impressed with one student, while others were not at all impressed. As a more seasoned academic, there are many different qualities in researchers that are important. Students, however, rarely see this. Instead, they compare with each other. This is a huge mistake.

I remember one professor who was very encouraging to me, despite the fact that I have smaller productivity. He said, look at the others' publications. How are they different from each other? Usually, there is just one idea or question for an entire dissertation, and this idea was generated by their supervisors. Subsequently, the professor told me that I hade the mind, independence and pre-requisites to become a great researcher in the future. This changed my "envy" of my competitors' productivity.

Furthermore, don't look ahead and make guesses about your chances to land a job in the future. You cannot predict the future, and hence you shouldn't worry that much about it right now. You can do that when your thesis is about done. My job prospects are still very limited, but I have created a new job that didn't exist before.

Also, we are talking about your education here, not the resources of your department or supervisor.

I would advice you to do a game theory solution. I would keep grinding the halls of science, but apply for a job that you find really, really interesting from time to time. If you suddenly land a great job opportunity, your advisor will understand that you got an offer you cannot refuse. If that happens, drop out. If not, graduate and be really proud of your accomplishment.

Finally, this is advice I give to all grad students. A thesis should be done, not perfect.