I build a Linux system with multiple NICs. It’s heavyweight for a router, and probably not very efficient, but it does the job very well and I love it. I’m using foomuuri to manage the NFTables and NAT and it was very easy (although it’s a very obscure piece of software and the documentation and community isn’t excessively comprehensive, you might have better luck with something more common but it works nicely for me). It can pull in all the adblocking lists and all that fun stuff. I find it pretty intuitive to configure, at least compared to the horrors of most networking/routing configuration nonsense.

For DHCP I tried using the ISC’s new “kea” in the hopes it would be modern and clean alternative to the venerable old isc-dhcp-server but it turned out to be a horrible mistake. They made some awful, insane, unjustifiable design decisions in that software. It gets confused about IP addresses already being claimed (by the exact same host/MAC) address because some arbitrary value changed in how it identifies itself, and then does absurd things with static leases, it just randomly decides to give it a dynamic lease instead, and then will fixate on that dynamic lease and never let it have the static lease back, I fought with it for months, it kept breaking my network for absolutely no reason and it was a nightmare.

Back to trusty old isc-dhcp-server for me. And of course bind for DNS, I’m a bit old-school but it’s very comfortable.

dnsmasq might be a lighter-weight and easier alternative you could consider, I think it does DHCP as well, convenient.

And of course, being a full Linux server means you can put all kinds of other stuff on it too. Stuff that needs to be both publicly visible and privately visible on the LAN seems like an obvious fit, but in fact it’s probably the worst place to put those services, because you run into issues with NAT hairpinning and other nonsense. For services that are strictly intended for the private LAN/VPN, or services that are strictly public facing, it can handle those just fine by restricting them to a specific interface. But you should avoid stuff that needs to listen on both sides of the connection, it can be done, but it’s tricky and annoying and not ideal.

If you need both internal and external access, port-forwarding from the outside through the NAT to another dedicated internal host is a much better solution if possible, it’s still a bit tricky (you may have to override the DNS for that domain internally to use the private IP instead of the public one) but it’s quite reliable and functional once you have it set up.

None of this is really for the faint of heart, it’ll likely require a fair bit of trial and error and physical access to the machine to resolve issues. But it’s a fun project and I think the end result is worth it.