iCloud+ custom email domain forwarding

I'm investigating starting to use custom domain for my iCloud e-mails but I wonder how to solve an issue I have: Some of the emails currently in use on my domain are from people not in my iCloud Family Sharing (I can't be added because of the 6 members limit ).

So the way I see it there is two ways to handle this:

1) One solution to this issue would be to be able to setup forwarding of those email to somewhere else while hosting the one form my family sharing member on iCloud.

2) The other one would be to use "split domain routing" which would require the "main" mail domain to not be iCloud but another one that would forward the email hosted on iCloud to a sub domain BUT it also requires iCloud to support this subdomain as an alias to the main domain.

I would be surprised if #2 was possible here but I wondered if #1 was supported, that is: Can I set forwarding to other e-mails that would not count toward my family sharing count ?