Seems like the easiest solution is to just make them real email domains and have them forward to your main one. With fastmail I just have all of my domains forwarding to one inbox and I send from one particular one.
Cloudflare can do this. Even for domains you don't buy there. You can use a free account. Their Security Center scans warn you of this and other configuration issues you may not consider.
Edit: Sorry, you did say "one click way." I meant the warning of these things can be done on a free account, but the actions taken can only be done on a domain bought through CF. I don't think there can be a "one click way" for this type of change because a misconfiguration can have a huge impact on your business if you're not careful.
I use cloudron on as part of my self hosted stack. During the process of standing it up I also had to stand up Google DNS. My domain registar is also Google. but anyways, I had to set up a service account and keys to allow cloudron to make DNS changes. So if it can do it, I imagine it just depends on your DNS / nameservers and their API and features.
