You are basically asking for people to solve a solved problem, there’s no actual need for keeping the PCs separate since you control them both, and oh and you want it done cheap. A bespoke custom solution will not scale regardless if you need it to or not, you should know that.
https://hometechhacker.com/great-choices-for-opnsense-hardware/
A firewall device with as many ports as you need is your best bet.
I do need to segregate networks but I do not trust the operating systems running on these switches which can do L3 routing.
Ok, so you are trusting the PCs which you need to keep separate.
There’s no way to know if one of them is hoovering all the traffic from the other, if they are both connected to the same unmanaged switch.
https://youtube.com/playlist?list=PLjVwd8FlHBASO5vLBtMYNOzm8Q9DegSjO
The more normies start using this, the more default config/ old as dirt routers will have some exploitable thing.
More than 10 years ago, I logged into the router of some guy on IRC and changed his pppoe username and password to 'pleaseinvestigateme ‘iamapedophile’ or something.
The IP he connected from was his home network, the router had default username and password. He disconnected when I hit save.
The guy was a pedo, fyi. Or trolling by saying he was.
quieter, and uses way less power.
VERY IMPORTANT PAY ATTENTION TO THIS SENTENCE OP
Not liking the solution you have doesn’t mean you don’t have a solution.
Anyway, watch the playlist I sent, it’s a great overview of the OSI model with some other stuff. You mentioned not understanding some layers, once you do you will understand the limitations of the hardware you have.