The proof that rendering is not _that_ hard because the flutter team did it when they switched off skia (although technically they still use skia for text rendering, I'll admit that text rendering and layout is hard)

How is a fact that someone did something proof that it isn’t hard?

I mean, every cross-platform commercial DAW manages to do it? Bitwig, Renoise, Reaper, even VCV.

Every space company manages to shoot spacecrafts into space, does that mean it's easy? Obviously not :)

Cross-platform native GUIs are still hard, although maybe not rocket science, but there is a reason most individuals/companies don't go for that by default and reach for other solutions.