We've also experience trouble almost every time we've updated our pools, ever since the old XenServer days and Citrix kind of recommended "manual intervention" because there was no mechanism to check which hosts that were suitable before a VM is migrated. I think there has been a lot of work done to XOA tho to handle this, but I might've been mistaken, we just ended up re-installing our hosts and setting up a new pool which we then live migrate our VM's over too and scrap the old ones. VMWare has some kind of logic, which will try to balance the load between the hosts and if you have DRS it will even make sure to balance your hosts automatically during runtime. Im pretty sure XOA has this logic as well, but XCP-NG center definately doesn't, so avoid it as much as possible.

The best way would be to use XO Proxies as "Reverse HTTP proxies" (or any reverse proxy in each DC) and then tell XO to connect to those proxies. This way, each DC will have only one entry point exposed to the outside, and you could manage that with your central XO. This is a subject we planned to work in the next months. If you have a support subscription, please open a ticket so we can do initial test inside your infrastructure

Indeed, XO doc was built for XenServer (before XCP-ng existed). @Darkbeldin will update this accordingly