Docker

Would a reverse proxy be a solution? I haven't done that type of thing in a while but maybe you could map different services to different subdomains and just route that way?

In your compose file, in ports, just set the first number to the port number you wanna use. I use 8088 for my nextcloud for instance. The second number is the number the docker container is set to use internally. So you can just set like 8088:80 in your compose file. That should do the trick.

Toss a reverse proxy in front like Traefik and that way any docker container can overlap however it wants.

One of the traditional ways to do this is to stand up a reverse proxy (e.g. NGinx) That then sits on ports 80 and 443 (you'll want TLS for NextCloud) and forwards traffic to those applications. If you are using docker for everything, you can have a back-end docker network where the NGinx container forwards traffic to the PiHole and NextCloud containers. And since each container is its own entity, you don't need to worry about mucking about with the ports for the different services, they can each have ports 80/443 on their own container and you don't need to worry about forwarding those ports from the host. Though, if PiHole is running on the hardware and not in a container, this can complicate things.