I love Fastmail, but this feels like a waste of time. I'd rather just leave Fastmail open in a browser tab, rather than install Flatpak just to load an Electron app with the web client in it.
Or if you gotta have a spearate app+:
Safari > File > Add to Dock
+ which I do because it's so much easier to go to my mail instead of wading through browser windows and tabs. I've been using that way for a while now
You'll still be able to do that.