Reads like you've picked the right stage to submit. Remember though, rejections don't mean you're not good enough. They just mean you're not the right fit for that agent at that time. And you've only tried a very small sample of agents.Several litopians have beta read the whole thing and said they liked it.
I had some feedback for things to work on from them; Some characters that were weak, the second half wasn't as strong as the first, some niggles that needed unniggling...
I dealt with them as best i could.
Overall, it was deemed to be almost ready (once said niggles were dealt with).
Honestly, three rewrites seems like overkill to me. I am, however, reading that as rewriting from a blank page, from scratch; if that's incorrect, I apologise. I don't think i could stomach starting from scratch again even once.
I've been through the manuscript a dozen times for a variety of different issues. (Proof, development, structure...) And on the last few goes through, I didn't change anything. Just read it straight through and thought "ok. This is it. This is as good as it's going to get."
I think I've probably reached a skill ceiling. I'm simply not good enough.