It depends on how much should be open source (wifi generally involves a lot of blobs), and how much the router should do more than just route, but a good starti

It depends on how much should be open source (wifi generally involves a lot of blobs), and how much the router should do more than just route, but a good starting point should be OpenWRT on supported hardware. I tend to separate the router and access points. Well working wifi hardware has traditionally been hard to find on consumer pricepoints, so I've used Ubiquiti thoughout. They're not open source at all, but neither are most network cards and wifi adapters so I consider them appliances. That way the router can be simple and separate.