What I haven’t figured out yet is whether or not I can give my docker services their own IP on my router for access from another system on a fixed or reserved IP.

You can. You have to set up a macvlan on your network and then assign an IP to your container that sits on your router's subnet.

I can only use traefik with a macvlan because Synology DSM uses ports 80 and 443. I assign traefik its own IP and use pihole's DNS to route wildcard subdomain to it.

I wrote a guide in my trillium notes. If you're interested I can share.

I see. Sure, that's a valid way to manage networking. I personally don't like to do this manually anymore, just like I don't drive stick shift anymore.

If you want to expose a service to the WWW I'd recommend using a reverse proxy. E.g. I use Traefik 2; it gets the config needed automatically from 5-6 labels per container and I don't need to bother with IPs, certificates, NAT and what have you. It just creates virtual hosts procures a LetsEncrypt certificate and directs the traffic to the target container completely on its own.
Spinning up a container and trying it out with its own subdomain with correct SSL certificates immediately never has been easier. (I have a "*" DNS entry to my Treafik server).

You also could try installing cloudflared and create a Cloudflare tunnel. This way you don't even have to forward any ports in your router.

Just some tips, if you want to explore new things :)