Do you really want to have a router and an AP? Why not go all-in-one?
Your NanoPi board is great, I used a couple of those and I’ve nothing against them. About OPNSense why?? Can’t you just run OpenWRT, what feature does it miss? People throw OPNSense like a good solution for everyone while in fact they don’t even use 1/100 of its features. You’re not running a large scale 100+ clients with dozens of APs then OpenWRT is more than enough. To be fair even at that scale OpenWRT would work just fine as well.
For a full open-source hardware and software experience you need a more exotic brand like those. The BananaPi BPi R3 and here is a very good option with a 4 core CPU, 2GB of RAM Wifi6 and two 2.5G SFP ports besides the 4 ethernet ports. There’s also an upcoming board the BPI-R4 with optional Wifi 7 and 10G SPF.
While there are things like OPNsense and pfSense that may make sense in some cases you most likely don’t require that. You’ve a small network and OpenWRT will provide you with a much cleaner open-source experience and also allow for all the customization you would like. Another great advantage of OpenWRT you’ve the ability to install 3rd party stuff in your router, you may even use qemu to virtualize stuff like your Pi-Hole on it or simply run docker containers.
Even of you don’t want something Wifi + Router all-in-one, the information above still holds. For instance the Banana Pi BPI-R4 is available on AliExpress for around 130€ and has a LOT more I/O than the NanoPi R6S.
About the switch, forget that crap, for the price of that link you may be able to get a decent TP-Link switch from a local store with that many ports. Like this one with some managed features or this dumb one. At the end of the day I would pick the first one cause it will provide you with good VLAN support.