On the other hand this app would work across desktop OSes as well, whereas Flutter not, as it isn't their focus.

Given past experiences, I am sceptical of using languages not part of platform SDKs for production code.

When I do use such third party languages then the eco-system plays a major role in the decision, and for .NET vs a language clinging for survival, the decision is obvious if I care about having the same code in production in the next couple of years.