> There are a bunch of existing extensions for Gmail UI tweaks, but most are just too heavy to me.

I went down the heavy customization path, and eventually came upon the profound realization that I was going the wrong way. I ditched all the extensions and went with the simple gmail[0]. No breaking changes to extensions over time. Less CPU load. Cuts out all distractions. Simple. Fast. I'm in and I'm out.

[0] https://mail.google.com/mail/mu/mp/

Personally I like to use Simplify Gmail (https://simpl.fyi/) which was made by a former Gmail lead designer and Inbox founder.