Need advice on networking with LAN-only PC - eviltoast

I need some advice on whether to get an additional 2.5 Gbps router, 2.5 Gbps switch, or both with the particular network setup I have. The current network I have uses a ISP-provided locked router (i.e. I can’t even change WiFi network channels).

https://preview.redd.it/hti0vjzrhe1c1.png?width=1744&format=png&auto=webp&s=29ecfa0c0d5152a5fabeb7327fd5bb63ccaebd72

The goal in this is to see if I can:

  1. Improve WiFi access to my security camera and improve WiFi speeds in my bedroom. (I tried using a separate WiFi 6 AP in-between rooms, but this did improve reception)
  2. Allow PC 3 a LAN-only PC to be accessible by other computers, but remain offline to the internet at 2.5Gbs transfer speeds
  3. Use my own router so I can change settings on my WiFi like network channels

The concern I have with my proposed option is that the ISP Router might be locked, so I really do not want to use AP mode on a new 2.5 Gbps router and be stuck with the same WiFi channel. On the other hand, I am not sure if it would be a good idea to use a 2.5 Gps switch before the ISP Fiber Router and then have that connect to a 2.5 Gbps router in my bedroom. Worst-case I can imagine, this would require me buying two 2.5 Gbps routers.