Having dealt with duplicate MACs in the past on a LAN, it’s chaos. Both connections can be affected. If they’re on the same network, it comes down to an ARP table race as to who gets to talk. If there’s enough separation between them that they don’t end up in the same ARP table someplace, it may have no impact at all. It will be similar to an ARP table poisoning attack.

Bitwarden for passwords
MXRoute for mail
Kagi for search
Backblaze B2 for offsite backups

I self host pi-hole, but I send them some money once in a while.

Same here. I do get a few emails about renewing my domain whenever it gets close to expiring. Maybe a text message, but no phone calls.

You might be able to put a google voice number in there. You won’t have to worry about it being taken away for lack of use.