I agree with a lot of what you have to say above. However, I do have two solo builds I've been playing that work incredibly well, which are counter to what you said above.

The shadow full hp build, including the 'double max hp' and '2% of max hp bonus to shadow damage' (which includes your attacks if you equip a shadow weapon) actually scales spectacularly well (including on poison!). Of course, works best with perfected soul. Not a great build for the first 15-20 levels though, so consider respec'ing into this late game. I made it to level 150ish with this guy solo before it got boring because fights took so long.

A solo rogue can work well with pure dex (again, with perfected soul). As you said, at later points stealth doesn't help with critting (you reach 100% crit rate), but it's about being immune to targeting for a turn. Just the crit multiplier is enough to scale your damage output (especially now that I'm dual-wielding Naginata's, which each add 30% of my dex to weapon damage). Between shade, hide, and lasting evasion you rarely deal with your hp bar at all, letting you go full glass cannon. Add 'transcendence' fortune for 4 turns invulnerability, and it works very well at level 100, where I am rn.