I feel/share your pain.
I have serious doubts wrt TB. A while ago there was a widespread encryption bug in mail clients and the only ones not affected were mutt and kmail if HTML-mail was disabled (by default). Thunderbird was affected.
https://mastodon.social/@nwalfield contains various relevant toots. Possibly (a bit) biased, but also knowledgeable.
Could be a (weird) coincidence, but recently got an encrypted email from someone using TB which I couldn't open; sth went wrong