Not sure about the capitalization, but I am liking this addon. https://addons.thunderbird.net/en-US/thunderbird/addon/grammar-and-spell-checker/ it might not auto capitalize, but it is a vast improvement over the standard spell check. While is is new for Thunderbird is has a much longer history with Firefox https://addons.mozilla.org/en-US/firefox/addon/languagetool/ I provide the link so you can satisfy yourself as to the history.

Looks like the addon does it while it does it's other checking. https://thunderbirdtweaks.blogspot.com/2021/08/grammar-checking-and-spelling-as-well.html