I actually just went through this because I previously used iCloud’s Hide my Email service, but I’m de-Appling so I had to find something new. I’ve used Hide my Email, Firefox Relay, and Addy.io.
Hide my Email is hard to replace because it’s shared with the entire @icloud.com address pool, so it’s hard for companies to reject and it’s hard to link all of your aliases together.
I ultimately settled on a combination of having a Tuta paid plan, which gives you a few email addresses, and Addy.io. I chose Addy because it’s run out of Europe (the UK specifically) and has some great power user features while still offering easy-to-use apps for all of the platforms I use. I’ve only had Steam reject an Addy address so far.
At the time, I also thought Firefox Relay only allowed you to use your custom domain, but they do allow you to use the more generic @mozilla.com domain as well for better anonimity. Firefox Relay does have a limit to how long you can reply to emails you’ve received, though (I believe it’s 90 days), it’s run out of the US, and I think Mozilla has done some questionable things recently with respect to privacy.
I didn’t check out SimpleLogin since it’s owned by Proton, and I didn’t check out DuckDuckGo’s offering because I believe it’s also a US company.
I haven’t had any issues with tracking (that I’m aware of, anyway) and their privacy policy is pretty straightforward (email storage is opt-in, they don’t share data with third parties except law enforcement and what they need to process payments). I suppose one could argue that you have to take them at their word, but that’s true of anything.
It’s hosted out of the Netherlands, which I believe has pretty strong privacy protections, though the developer is from the UK which fares marginally worse in that regard (again, grain of salt with my opinions here).
I’ll confess I’m also not a technical expert in email systems, but you can use your own PGP keys for what that’s worth, and if you’re super concerned you can always download their server runtime and self-host it as well.
I probably wouldn’t use it for my bank, mostly because it’s another moving piece that could break, but that’s why I combine it with Tuta - I can have a “real” address for my bank, another one for my doctor, and a few others for anything that might reject an Addy address (and that I can’t live without), and then Addy for everything else.