Great to read about someone in a similar situation to me. I work as the developer and maintainer of a niche-market financial / real-estate application. This application has been developed and supported since the late 80s, first being done in Turbo Pascal, then Delphi, and then under my stewardship we moved to C#. I refactored the calculation and report production code into a library, and since that time we've built a Mac version and Web version, all utilising the same 'core' library. This means that for critical calculations and data output we - my business partner, who is the 'domain brains', and I - can do all the hard work on the Windows version (with which we are most familiar and comfortable, and IMO VS on Windows is still miles ahead of VS on Mac), and then 'just' do the GUI work for the other versions.

We did look at doing exactly as you said, i.e. using a web view within Windows and Mac, however I couldn't really get things working well enough at the time (as TBH I am bit of a noob WRT web development, and just pick things up as necessary as we go along).

For our market, there is strong demand for the desktop versions, and this is even with a subscription model; people get access to the most recent major and minor versions of the software as well as phone and email support while under subscription. When their sub runs out they are entitled to minor version updates, but nothing else. My biz partner is very good with people and very knowledgeable in the domain we operate, so this kind of arrangement suits everybody. Oh, and I get to work remote, and have done with him for ~15 years. The current situation really makes one appreciate fortunate arrangements such as this.