Agreed. The UI is painful, but the project priorities are correct: fixing TNP and fillet/chamfer needs to come before the polish. First make it minimum viable for the non-zealous, then we can talk about drawing attention with a sexy dark-mode-by-default UI overhaul. It's getting close, but that makes this the most important time to exercise restraint.
Alas, fully fixing the issues with fillets and chamfers may be impossible, because those specific problems are quite foundational issues in OpenCascade. I don’t know if it’s even possible for FreeCAD to know that a fillet would fail by testing it, because of limitations with the responses that come back from the kernel.
It might be possible to predict which edges would have problems, but I think that would involve repeating geometry calculations outside the geometry kernel?
Specifically the main problem with fillets/chamfers in OCC is that the new face cannot completely consume/replace an existing edge.
That is to say -- I think!? -- that one of the edges of the new fillet/chamfer is always an existing edge just displaced, and no edges are destroyed. If an edge would be destroyed, then it fails?
But this is just my colloquial/folklore understanding.
There have been newer versions of OCC that FreeCAD has not yet switched to, I think. (And there is a new (Chinese) fork, called OpenGeometry.)
So maybe there are some fixes already in the pipeline or in the future. But they are somewhat out of the FreeCAD project's control, is my understanding.
I definitely agree on it not being intuitive. Something where I likely will spend hundreds of hours of work and learning is something I want to be unambiguously available to suddenly being taken away from me or be behind a paywall of thousands of dollars. You are 100% correct though that this tradeoff isn't for most people.
But there’s no denying that FreeCAD is still a bit of a lifestyle choice :-) I love it, but I don’t try to sell everyone on it yet.