At 10BBs and lower I resort to Nash shove/folds. I know this is not optimal, but I don't know what the perfect strategy is. I wish I knew what it was, but I don't. At 11-15BBs I'm limping a lot more. I think because of my shoving at 10 and lower, people see my limps as weak and jam over them.

I struggle from the BB when people keep limping at 15-10BBs. Should I be jamming all my aces vs. these limps? The problem is a flop comes AT3 and I'm pretty much having to fold everything, because they have so many aces, but I don't because I'm jamming them preflop. I've occasionally checked back with weak Ax, but usually I'm jamming them. Should I always be jamming them though?

With Kx, I'm usually in a pickle as well. Esepecially when I'm at 15BBs and I'm holding K7o. If I limp and they jam, I'm not sure if I can call. I usually fold just because people usually have Ax and pocket pairs. If I raise and they jam, I'm not happy either. Weak Kx is something I don't really know what to do with. I don't like jamming either, because they don't call with worse that often. I'm guessing the best strategy vs. most people is to limp and call a NAI raise, but fold to a shove.

In general as you get shallower you should be jamming more of those hands over limp. At 15bb you probably want to check back some aces, but at 10 you should be mainly jamming.

Meh Kxo are definitely fine to mainly limp and then fold vs jam. You can't call jams. What you do vs iso depends on your hand and the villain.

Im assuming you are talking about playing fish so all that matters is highest ev action.

You could look at your database, check how your hands are doing when you check back, this is usually hardest part, but from your post I can tell you check back a lot, so at least you will have some sample there. Than try to figure out how same hands will do when you iso jam. Start looking how much people limp/fold, what ranges they limp/call etc. Do some calculations.

Id say there is no way you will ever do better checking back Ax than ISO jamming, even like 20-25bb deep. Id also suggest looking in to Kx.

About Kx yes, limp/fold to jam,call nai raise will be best, limp/folding to any raise bottom part of Kxo.

These are very good questions. And you can answer a lot of them just by studying your database.
Also dont be afraid to get raised when you limp, you are supposed to get raised. gto iso 40% 10bb, thats almost every other hand and gto SB still plays almost 90%, limping majority of its range

When my opponent's limp I usually jam over them with the same hands I would have called with had they jammed. Oddly enough at 10bbs that turns out to be 39.37% of my hands. On the downside though, my checking back range is super weak, so if any high cards come on the flop, I'm either making heroic calls or folding a lot.

I feel like my opponent's are adjusting to me now and I no longer have an edge. When I started I was winning 50% of the games I play. Now I'm barely hitting 33%.

I need to develop some stronger fundamentally sound strategies rather than trying to exploit everyone, because they are playing too balanced. I used to play $50 HU SNGs a few years back and would make easy money, but now the HU part is where I struggle the most because people 3-bet relentlessly and I don't know GTO.

You def can go wider with iso than os calling range, for example, adding low suited connectors and Kxo.

Yes your xback range will be weak and is supposed to be weak and thats fine, dont forget people are likely to limp weak ranges too.

And on the last one you are totally wrong, forget about gto, study your database, check % of different ranges, check ranges to get a feel what hands are in those ranges. And do a shitton of calculations.

Like in your last example, you can use gto ofc but lets say guy is actually 3betting 40% (99% of the time people actually 3bet less than you think or your hud says), gto would raise/fold something like A6o which would be huge mistake vs this guy and raise/fold a lot of other hands which vs this particular guy makes sense to limp or raise/call for example.

Check what you should raise call vs 40% 3bet, 35% 3bet, look what if its top30% what if mixed ranges etc.

gto is not an answer, I remember reading somewhere in freeqone thread where he discovered that playing gto vs fish is bad, which is obvious and using gto preflop ranges vs fish is especially bad.