Have I 'spent' my negotiating power on higher salary, or can I further negotiate the TT startup offer?

In general, departments don't like extended negotiations with candidates for faculty positions. In some cases, candidates use this as a way to extend the time that they have to consider an offer so that they can interview for other positions and obtain other offers. If a candidate ends up not taking the job after an extended negotiation then it may be too late to get the next candidate on the list.

There are differences between salary and startup packages:

In negotiating faculty offers the budget that pays for faculty salary is often different from the budget that pays for the startup package. In any case, the startup package is one-time money rather than a permanent salary. Thus if the one-time money is available it might be easier to come up with $100K more of startup funds than $10K per year of salary.

At public institutions, faculty salaries are often public information while at private universities these are generally secret. At public institutions, it can upset other faculty members if a new assistant professor is hired in at a much higher than normal salary. Large differences in salary are much more common at private institutions.

It appears that the OP made his salary needs clear before agreeing to interview but that the startup package was not discussed before the interview. Furthermore, the offer that he has received is the first offer from the institution. Under those circumstances, it would be quite normal to negotiate the startup package.

Most departments do not want to negotiate piecemeal. They generally make you an initial offer that includes salary, benefits, space, teaching, startup package, etc. Most applicants then respond back with a counter offer that includes improvements in one or more area. This can go back and forth a number of times. I have never seen, or heard, of anyone trying to negotiate an offer one piece at a time.

I would therefore guess if you spent time negotiating your salary and not talking about the other aspects of the offer, that the department was assuming that those were acceptable. In other words, yes, you most likely spent you negotiating power on salary. You can always go back to the table and see if you can get more, or what you can trade reductions in your salary for.