Slitherine

In terms of gameplay and ease of finding opponents (not just tournaments), what games would you recommend? I already have Battlesector, Panzercorp 2, and Southern Storm. I enjoy all of those but I want more

Field of Glory II - both Ancient and Medieval versions. Lots of player made leagues plus the official Slitherine tournaments.

For WWII Valor and Victory is good or Combat Mission. All these are more tactical games than you listed but also good MP games. You could try Order of Battle for a more operational MP game.

Have you considered the Strategic Command titles?

Field of Glory II