Since only 1 dB is allowed for always on I have to setup other 4 db’s in db mirroring.
That makes absolutely no sense as they work basically the same way. Yes, there are some very real differences between them, but if someone told me "this works for mirroring but can't be used in availability groups" I'd ask why. the capture mechanism is roughly the same, the transfer mechanism is roughly the same, and the apply mechanism is roughly the same. When I say "roughly" I mean they are not 100% identical.
I'd put them all in AGs.
My always on endpoint is already using port 5022. Is there a place in db mirroring where I can change 5022 to let’s say 7022?
No, because there can only be a single mirroring endpoint which is what both availability groups and mirroring uses. The reason that it's called a "mirroring" endpoint and not an "availability group" endpoint is two fold. First, mirroring came first. Second, if the name was changed it could cause a compatibility issue with older applications and code where the keyword would change, so they won't change it due to backwards compatibility.
I'd be happy to understand why someone is saying that a database can belong to mirroring but not belong to an availability group.