If you're comfortable running your own router, my suggestion would be to install Opnsense on the new celeron box (as long as it has multiple ports and all the drivers exist in FreeBSD) and keep the TPLink in AP mode so it only handles the wifi side of things Opnsense is incredibly powerful and should have no problem running as your DHCP/firewall/wg box. I don't run pihole anymore since it has an Adguard Home plugin you can set up, but I did find it a bit more challenging to configure than pihole was
I have an alt account on sh.itjust.works for whenever lemmy.world is down. I'd love to know what you dislike about them so I can determine whether or not I should have a different alt instance. I'm pretty OOTL when it comes to instance drama