If those games can do LAN connections (computers on the same network), you could use something like Hamachi to put your computers on a virtual network together. I'm sure there are more modern versions of Hamachi, but that's how I used to do it in the early 2010s.

For all their faults they don't use kernel level anti cheat. Both StarCraft 1 and 2 are free. StarCraft 1 should at least have LAN play.

OpenRA comes to mind.

I'm playing Age of empires 2 definitive edition from Steam and it's an awesome modernized RTS experience

I cant help you with RTS games, but u should def try old wow private servers. My recommendation is Turtle-wow . Its old school wow with tons of players aaand because its old, every pc can run it.

Parsec is probably what you need. Me and my brother played through BG1 (for ps2 I believe) with an emulator and parsec. For the most part there was no lag. We simulated us both playing on his computer the same way you would play couch co-op, just in different areas.

You can use playit.gg or localtonet.com for making lan game reachable from internet to your friend and vice versa. For Diablo 2 Resurrected there's a mod that allow to play on LAN (original blizzard version doesn't have LAN) from here - it's works out of the box. Here You can download original Starcraft 1.16.1 version installer. Works very good on LAN. Good luck!