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!
At first I thought I'd like it as a standalone app but I would much prefer it to be bundled with Thunderbird since it's one less app on my phone and I know people talk about it locking you into one ecosystem but you can choose what email provider to use in Thunderbird. It's not like your locked to only using gmail in Thunderbird lol. I find having less apps on my phone frees my mind up and combining calendar into Thunderbird also just makes sense to me personally.