How to make totally self-sufficient Fallout 4 settlements without fast travel

Your question is based on a false premise.

Time passes in game when you fast travel. Not only that, but it can actually be faster to run there.

Keep in mind that there are still fast travel options in the survival game which you can use to speed things up, even though you can't just teleport anywhere. You can use Brotherhood vertibirds after clearing Fort Strong for Elder Maxson. These are faster than running, so should be even faster than fast travel. You can also teleport to the Institute after finishing Institutionalized, but can only teleport back out to the CIT Ruins. Still, if CIT is closer to the settlement than you currently are, this can save you a lot of time.

Furthermore, in my experienc,e failing to participate in a settlement defense often has little to no noticeable penalty, assuming that your defenses are sufficient. You may need to visit and repair a few things.

After a couple days of play time on survival, I don't find managing my settlements to be any more difficult than it was previously

Patch 1.4 has changed how defending settlements against attacks works (even if slightly). You will now receive a notification showing the result of the attack if you decide not to participate in the battle. I usually participate in all battles so I have yet to see a notification like this.

I was once stuck in the middle of a long quest when Sanctuary got attacked, I received the FAILED message (this was pre 1.4) approximately 1 hour after the attack began, so could this be the standard?

I also never use fast travel as, in my opinion, I make many more discoveries and pick up tons more valuable loot when moving along the map myself. And as I said, the only times I miss settlement battles is if I'm stuck in a long quest.

By the way, I have 14 settlements so far, all with 80+ happiness. Just make sure you're producing enough resources (food, water) and have enough beds and turrets and the settlement should grow nicely without much intervention on your part. Make sure they are all connected with Supply Lines so you can share building materials between them in case you do have to stop by and build more beds, crops etc...

Don't forget that you can check on your settlements using the PipBoy in the DATA section under WORKSHOPS I believe (although it is a little buggy at the moment, it will give you an overview of what needs fixing and where).

Settlements are a great part of the game in my opinion, they give you places across the map to dump your junk, fill up on purified water (excess water produced will end up in your workshop) and when Survival Mode gets released, you'll have save points as you will only be able to save progress when sleeping in an owned bed.