I ended up just keeping my ISP router as is, and putting a DD-WRT router behind it. I suspect the double layer of NAT isn't great, but I've been doing it for years without issue.

DD-WRT has been pretty solid for that, even in non-ideal network setups (currently have it connected to the ISP router as a 2.4GHz client, then acting as a 5GHZ AP for my other devices).

Use a separate modem and router. If you can disable NAT on your current combo unit then you already have the modem.