When I went searching out options like you I found many things and I was sure I was going to want to use OPNsense, it was what I felt the most used by youtube people and I thought it was a fork of a proprietary pfsense. When I did a little more research I found out why OPNsense was used by so many Youtubers, OPNsense is proprietary-ish software for certain firewalls. The next thing I found was openWRT, once I found it I went searching for the best hardware for it and found the Flint 2. After I got that I flashed it with a vanialla install of openWRT.

My original plan was to self-host an email server, so I had gotten 2 static IP addresses. They were free so why not I thought. Turns out hosting your own email server is not that easy, so I went to the company hosting my domain. This is all besides the point.

After getting my static IPs and my Flint 2 setup (I did it backwards you should really have a firewall before the static IP it was not to far apart but it would have been easier to do it the right way), I could do things I could only dream of in the past. I could have a VPN into my LAN (using wireguard) so when I was out and about I can still use my pihole setup to block trackers, ads, and some spying eyes. I have vLANs galore and if it does not need to be attached to the internet it is not, or it is in my IoT vLAN that does not have WAN access.

In short do not think it is simple, it is not. The ISP router does so much more than routing it is also a sad excuse of a firewall and it is a modem.