I use Owl so that I can use Thunderbird for my work emails, it's paid but it's cheap, I think it's about $10 per year. https://addons.thunderbird.net/en-US/thunderbird/addon/owl-f...

Expression search is good for improved search. https://addons.thunderbird.net/en-us/thunderbird/addon/expre...

This one allows you to @mention people. https://addons.thunderbird.net/en-US/thunderbird/addon/menti...

This one gives you a confirmation dialog when you send, closest thing I could find to Gmail's undo. https://addons.thunderbird.net/en-US/thunderbird/addon/flex-...

This one gives you a conversation view. https://addons.thunderbird.net/en-US/thunderbird/addon/gmail...

I have multiple email accounts and the "Correct Identity" plugin is useful for setting rules to ensure I use the correct account for different recipients.

I use darkreader to make emails light-on-dark.