Proton doesn’t offer its own aliases, they use a third party service as well. It would basically be very similar to what you already get from Relay.

Have a look at Mailbox.org if you’re looking for integrated services, they offer packages with more than just email. It’s a long-running German service.

Please be wary of “encrypted” mail services, they make it fairly hard to migrate away from them later, if you need to. You need special tools to get your mail out of them, and those tools are at their whim.