I've been running a PC Engines APU2 (https://www.pcengines.ch/apu2.htm) for many years now. It has enough power to route over my gigabit fiber connection. It has mini-PCIE slots for wifi cards. If you want 2.4GHz and 5GHz bands, you'll need two cards and run two instances of hostapd.

The software stack is pretty basic:

- IPtables for routing and firewall

- DNSMasq for DNS cache and DHCP

- Hostapd for wireless