One thing to look at if you’re going this route is whether your router supports NAT loopback (a.k.a. NAT reflection or NAT hairpinning). This feature means that you can access your server via the external IP (and therefore via the ddns domain name) even from within your network. It’s really useful for phones and laptops that might be on your home network at some times and off somewhere else at other times, so you don’t have to change configurations on e.g. the Nextcloud client, or remember to type in different addresses inside and outside the network. Some routers just do this, some don’t, some it’s a setting you have to turn on. The router built into my ISP-supplied cable modem didn’t support it so I got my own router and put the ISP one into bridge mode.
For me (I use Kavita) it’s because I want to be able to just pick up whatever device is in front of me at the moment and pick up the book where I last left off even if it was on another device