I feel the same. I have "only" about 15 years worth of mail in Thunderbird but I am getting increasingly tired of working with the GUI, at least for reading and searching historical mail.

One of these days I will convert my mbox mail to Maildir and try out notmuch.

I switched to alpine (https://alpineapp.email/) after 20 years of thunderbird. Converting 20 years of archived thunderbird emails took a couple of minutes with a script I found somewhere, so now everything rests securely in the Maildir format.

I am quicker, happier and more efficient with TUI email than ever before, and as a bonus, the memory and CPU footprint is too small to see.

What is important though is to keep in mind that I am not a power user. At most I might get 200 emails per day or so, but for that work loads everything works brilliantly.

I have automated rules, shortcuts, I can define my own keys, I can even rewrite the source for smaller things if I want.

I recommend everyone to invest a day or two just to see if it is for you.