I pulled hushmail, proton, and gmail x 2 into thunderbird for quite awhile. Gave up on proton as it required their bridge to run background 100% of time, and the paid subscription wasn’t cheap. They double the price the 2nd year.
I’ve stuck with hushmail as my confidential one. The gmails are for work and non-deplorable stuff.
hushmail looks promising
I’ll let you know if I choose it.