I don't know why the OP has such difficulty. Both thunderbird and thebat support oauth for gmail- at worst gmail still supports app passwords.  Not sure if thebat supports it for microsoft. You do have to get the imap(pop) and smtp servers and correct ports for either to function properly. This is generally specified by the email service used. Probably where the OP messed up.

I'm not directly familiar with fastmail but do know someone that uses pobox (which got bought my fastmail not too long ago). He's on some $100 plan and stuck because the email address is well established (over 2 decades). Frankly, even at the $5/month price point, I would establish my own domain and use some cheap webhost to host the email services. Plenty of cheap shared hosting services at less than $50/year. With this arrangement, you can set up as many mail boxes as you want, configure a catchall for your domain.  The latter is great. I make up emails all the time on the spot. If an email gets burned, no problem, the real email is never revealed.  


Many years ago I made the decision to establish my own domain. This was the best decision. I'm not tied to any particular provider for email services for any reason. Domain name and services are managed through different entities so one doesn't have a stronghold on the other.


I do agree, no one should use gmail.  It's a privacy nightmare. However, as a cloud service, any service that processes your email will have access to its content including paid services.

Given the issues the OP has with standard email config, i highly DO NOT recommend the following.  I still value what bit of privacy remains. As such, I am hosting my own email server. Fortunately the isp was kind enough to open up port 25 in/out so the local MTA is able to function. Been doing so for the last 3 years. Other than a missing PTR record, everything else is in place including spf, dkim, dmarc, tlsa, dane and something.


As for email clients, I switched to thunderbird about the same time I set up the above server.  Long time thebat user for 20+ years. It was getting long in tooth by not exposing some underlying settings I wanted access to - ie, like not embedding the local hostname/ip in sent messages, or at least allowing the option to spoof those.  There were some other things too that I can't recall. Search was simpler. However, with the right mix of addons/extensions, thunderbird is operating quite well.


For mobile devices I use aquamail and have explored fairemail. Both offer a huge assortment of configuration options with the latter really digging into the granular aspect of config - enough that even I'd say it's overkill for a mobile app. Aqual mail offers the right mix (for me) of configuration options while still offering much more than the simpleton gmail app.