Each subdomain is on a different server on the client premises. So combining them in 100 is not an option. If one gets compromised all other are compromised which breaks security.
I’m using dehydrated on a cron job on each server so for your proposed workaround I have to calculate with each next new server to start earlier than a server already issued certificate. This is extremely complicated to be reliable solution.
But even if I do manage to make it, It still would not work, cause I would not get an empty slot.
I have issued approximately 3 certificates per day, so even if I manage to issue a new one before the renewal, the renewal after it would still fill the slot for the next day, and the next day and the next day.