I was in your exact same boat a year ago, and decided to try both solutions. I ended going with the streamlined one. As you said, you are already paying for the service and Proton Pass is imo a pleasantly nice password manager to use. It is a lot easier to create and delete aliases there than through Bitwarden/DDG, at least that was my experience. Proton pass is now my most used day to day app list and I'm very happy with it.

While all eggs in one basket isn't great from a security stand point, I am pretty happy with this solution. I do however keep 2fa in separate app, Ente Auth.

A custom domain is $12/yr, and SimpleLogin lets you do automatic regex emails, so I can just make a quick website.spam@customdomain.com email for each website. Would recommend.

Proton with a domain you control and use their Simplelogin which you can self host down the line should there be a rug-pull event. I think you need to manually export this so make it a habit as you add them!

You can put your eggs in one basket, just make sure you have a plan B if the basket catches on fire, using their domain in my eyes you're going down with the ship, if you control it you're just repointing records to a new host and getting simplelogin going.

This is part of the reason I like to keep ALL of my emails on disk still as well, if you can't decrypt your mailbox for some reason they're about as good as gone.

I'm a fan of separating services when possible.

And emails are a huge pain to change, so it might be worth considering an email service with your own domain name.

I think both solutions are probably reasonable secure. Personally I use the proton solution, because I like that my data is in Europe and I prefer to pay for services. I like the more transparent and honest business where I pay and get something in return.