My suggestion always: design all the playing nations by yourself... yours and the enemy one(s)
You play them for some turns to recruit some elite units/research some critical early spell/ conquer some easy indies, and then turn all the ones you don't want to play AI. It may seem boring, but you'll maybe find yourself continuing to play those nations more often than you imagine
Of course you'll need to avoid blesses or SCs in need of equipment for the enemy nations, as those aren't well managed by the AI later. For them, go for good scales and/or strong mages with national paths, researchers, or natural SCs like dragons, manticores/4 armed gods etc
