I've been using TB for a decade and I too can't find anything better (even if my use case is very simple).

However, I find TB's development very misguided - it's evident to me that they give very little priority to stability:

- addons support (APIs) is a dumpster fire, and IMO a large addon ecosystem is what makes a client unique

- not so long ago, they added an instant messaging client, which has been a waste of dev resources

- at some point they overhauled the UI, but the result was a bloated slow mess (on some platforms), even with broken defaults

- there are bugs open for at least a decade (I consistently hit one)

It gives me the impression that the management prioritizes work that looks good on a screenshot, rather than stability.

I think it'd be positive if the Thunderbird org shut down. There are more pragmatic teams who could take over the project (see Betterbird).