Thunderbird search is bad enough that I just open up the Gmail website to find an email. At this point I don't even know why I use a local email client, except maybe 25 years of muscle memory.

I kindly disagree. Yes, Thunderbird's search is not revolutionary, but when I direct it to a box containing ~20 years of e-mails, it returns instantly, searching for the words that I want.

...and I'm not even downloading the e-mails to the system to save disk space. I have explicitly disabled that.