It would be nice to have and I would happily pay £4.99 for it, which is the same cost as DavX5. Although since I bought DavX5 I'm able to connect my CalDAV accounts to Google Calendar for Android, so that has solved the problem for me at least.
I don't like feeling imprisoned in any specific tech ecosystem either (I'm also moving away from Proton, little by little); it just seems to me like dedicated apps work better than those designed to work with many different ones.
I just discovered this Android app called KashCal that's on F-Droid; it does a really great job syncing calendars from Fastmail to Thunderbird on the desktop and on mobile. Since TB seems to lack this ability, maybe they could join forces with the developer just like they did with K-9 Mail. Just a thought!