Consider self-hosting HALF the service. Something like this:

Outbound

local Postfix on dynamic IP → relay (optional) → recipient

You can configure Postfix to use a relay depending on the recipient. E.g. if you need to reach alice@outlook.com, MS will reject your dynamic IP. But if you have bob@outlook.com, you can tell Postfix to relay via MS servers using your bob@outlook.com account for all *@outlook.com recipients. And yes, you can still use a different vanity address in the FROM: field, like Gobbel2000@nerds.org, if that’s what you want to be known as. You can freetype whatever your want as the from address if you use a good MUA like mutt.

Inbound

(your acct @ rise-up or disroot.org or danwin1210.de) → POP3 onion → local Postfix → dovecote or procmail → local files read by your MUA of choice

You avoid a lot of complexity and labor by not maintaining a WAN-facing server. Though you still have a fair amount of effort in configuring your junk, you need not do all the configuration up front. You can do it on a per-outbound msg basis to spread your config effort out over time.

You can even hack postfix to send over Tor. And you can make it possible to support *onion email addresses, which is something that no non-self-hosted service offers.

When I email someone for the 1st time, say it’s alice@someunknownneverseensvc.xyz, I first configure my mail server to relay to @someunknownneverseensvc.xyz over Tor. If that fails (and it often does), I configure Postfix to directly send to that server from my dynamic IP (or VPN if I have that running). If that fails, then I can cave in and compromise my privacy by relaying through a 3rd party, if I choose. If I really want to send the msg but I really do not want an additional MitM, I may be able to create an acct on @someunknownneverseensvc.xyz and use that as a relay.

This approach relieves you of the reliability problem… you need not maintain a server always online and listening. But of course you lose some privacy because all your inbound traffic is seen by your ESP. At least you can potentially cut out your ESP on outbound mail.