I think the NanoPi range (https://www.friendlyelec.com/index.php?route=product/categor...) has great hardware for making your own router. They support various flavours of Linux, including OpenWRT (or at least their branded version called FriendlyWRT). I like the NanoPi M5 model as it supports using a NVMe so it can happily run some Docker containers on it. The case is really well made and uses passive cooling, so it's ideal for a router.