It’s a good idea. Also, domains are pretty cheap so I feel it a worthwhile yearly expense.

I’ve used addy.io for years and never had a site reject my alias. But I might do things differently than you. I own two domains for emails. My main one that is configured with Mailbox.org. And my secondary domain has its MX records set for addy.io. So when I create an alias I just make it at service@domain2.com. This is probably why I’ve never seen a service block me. I have seen lists on GitHub for companies to use in their code to block alias emails, and addy.io was in there. Are your aliases generated using their domains?