I have posted multiple issues about Archer C7 here, but I still like it, so I want to see if it can be improved with some extra hardware that I already own. So here are few things I want to try. Is this going to work?
Qualcomm 2.4 GHz dies after some time. Can I connect a Realtek USB wireless adapter and use that for 2.4 GHz? It is N300 USB adapter that works on a linux desktop. RNX-N250UBE or another powerlink RTL device.
Hardware offloading: I have managed (VLAN compatible) switch. I tag my three VLANs to one port on the router that connects to the switch, and use the switch to connect different devices. Will this improve the LAN speed/C7 load?
no. Realtek USB wlan drivers don't support AP mode - and 24/7 stability of (including non-Realtek-) USB wlan cards is even worse, much worse, than your internal one.
Not at all, the internal switch can do line-speed easily, the performance is lost/ needed to connect from the internal switch to the SOC-internal CPU and to do the actual routing, using an external/ managed switch won't speed that up in the slightest.