Monte Carlo with a double-dummy solver is fundamentally insufficient for good single-dummy play because it is incapable of understanding information. It won't take discovery plays (lines aimed at discovering more information about the opponents' hands before choosing a line of play) and will systemically overvalue positions which are good double dummy but require a guess. It doesn't understand falsecarding (because double dummy, it doesn't matter).
I agree that many games could make progress if people were actually inclined to try.
I think it will get a bit better in the coming decade thanks to continued hardware improvements & powerful LLM coding agents making it more feasible for amateurs to tackle these things at home. Personally I've been working on a game AI project for the last month at home based around published techniques for a similar game, using my 5090 for training and Opus for implementation and orchestrating tasks and so on. It's going quite well and it looks like I'm on track for a SOTA, superhuman AI at the end. Doing this ten years ago would have been incomparably harder.