There is a standard called Domain Connect [0] that attempts to achieve this.

Mailhardener will auto-detect Domain Connect and offer it automatically for 1-click configuration, but your DNS service provider has to support it.

[0] https://www.domainconnect.org/

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.

Not exactly one-click, but define your DNS records in Terraform and copy them for each domain.

A terraform module would be nice.