I found the request a feature link to suggest this exact thing with the exact reasoning you have.

It's disappointing this doesn't have more traction in almost a year, but I still give it my vote. I wish I could vote for it 10x

sites are now starting to block passmail adresses so this is becoming vital

While I really enjoy the possibility of using subdomains or custom domains, I don't want to lose the randomized *@passmail.net possibility. Ideally, there should be a setting for default alias creation for Proton Pass as well as a checkbox to possibly manually override it when creating an alias.
Any step towards being able to use both, subdomains/custom domains AND the randomized *@passmail.net alias from within Proton Mail Security Center (on the right with other aliases) would be great!

The suffix should be controllable under the "Advanced options" in the alias creation screen. However you can't set a default suffix/domain at the moment.

You may check out this discussion as well.
https://protonmail.uservoice.com/forums/953584-proton-pass/suggestions/46868950-ability-to-select-default-alias-domain

Currently, when normally using the Random Email Alias feature, it's randomized per website! For example:
Facebook: facebook.blashpomy29483@passmail.net
Spotify: spotify.funtime492927@passmail.net

This type of randomization is why it's more secure. If you use a Custom Domain, this option is not possible so long as you have the custom domain linked to your account (thus SimpleLogin account).
So now, it will be:
Facebook: facebook@domain.com
Spotify: spotify@domain.

There should be a checkbox to make it randomized or not make it randomized. As with this randomization (website.word-numbers@passmail.net) it is more secure.

Using a custom domain for hide-my-email aliases would help address this request, I think.

This feature is available with Fastmail and I just transferred from Fastmail to Proton and I am very surprised that I cannot use my custom domain for the Pass-generated random email address prefill. For example, If I used a Fastmail email address with Epic Games account, they would deny it and they would not let me change my email address. I then used a masked generated random email address for my custom domain, and it was accepted by Epic instantly. Time will tell in 3 weeks if I have issues changing my Epic Games email address with the passmail.net domain. I would love an update on this issue as I feel it's incredibly important. What would organisations do in this instance? If they wanted to use their own domain? I am already paying a premium price for Proton and feel this should be already included.