Gen X here and it has been common knowledge since 2000 that the only way to not fall behind your peers in terms of salary or career advancement is to change jobs every few years.
Existing staff getting paid less than new hires has been a thing for at least 25 years.
You can still do this but as others have said you need to have 2 separate lans. Your old Lan can go to PC 1 from old router. Then opnsense wan goes to your existing Lan and importantly you are now creating a new Lan on the lan side of opnsense. Here you can connect the PC 2 to test with. Each PC should only be on 1 Lan and each Lan should have a separate subnet.
Your diagram is weird. Isn’t the opnsense box supposed to replace the router. Or at least it should be between the existing router and you clients. Pc 1 go to opnsense Lan. And opnsense wan to the router and internet.
You’re creating all kinds of loops which is generally a bad idea. Your data should flow in 1 direction like a tree.
Unless there are a lot of details you’re not sharing.
Also remember generally a router is not a switch. Plug all your PCs into a switch. Plug a wifi access point into the switch. And then have the switch go into the lan of your opnsense.