Sips'

Wasn’t aware of their price, but yeah thats affordable! Never checked posteo put for that exact reason tho, custom domains is an absolute must for me.

Check out Migadu. Its by far the most affordable option out there I’ve found, at 19€ per year. Its also highly customizable but it does require a custom domain. Highly highly recommend!

It is extremely easy to switch email providers these days. I’d suggest start by setting up a forwarded mail box between Gmail and whatever new mail provider you choose. Then slowly but surely start using the new email address instead of Gmail and change the most important accounts to the new one.

From my understanding thunbird is somewhat separated from this. From the article linked by OP it says:

What’s crystal clear is that Thunderbird’s ever-increasing donation revenue (currently its sole source of income) is allowing for some explosive growth that’s long overdue. To add some context to this, Thunderbird received $2.8 million in donation revenue during 2021. Two years later, in 2023, it received $8.6 million in donations. I’m told that total financial contributions for 2024 were even higher, though the final amount hasn’t been officially released.