What are your suggestions for a technological stack for the described cross-platform setting?
Many people underestimate the power of webview based tech in mobile. For most usecases, webview based tech does just fine.
I have tried it myself, but did not love it fwiw
Flutter also does support Web but has a few issues. But still I wou choose it.
If you really want a desktop app, I would recommend using React for the web, React with Electron for desktop, and React with CapacitorJS for mobile.