A different kind of steppe system

[Publius Quirinus]

I'm not sure if this might work, but hear me out. In steppe regions:
[1] have a hinterland building (maybe called 'Steppe Region') that reduces the tax rate and trade significantly (-80% and -50% or something)
[2] perhaps some sort of pop. growth penalty
[3] reduce the upkeep of Scythian (faction) steppe units to very low or nothing
[4] reduce the recruitment cost

Of course, these low-cost steppe units can only be recruited in steppe regions. And maybe regions further from the coast and further to the northeast will have larger penalties than nearer regions.

This way it becomes prohibitively expensive for other 'civilised' factions to colonise the steppes, as garrisons (and AOR units recruited within the steppes) would still cost normal upkeep without bringing in any income to speak of. Possibly there could be a late-tier building called something like "Steppe Pacification" that requires the highest colony level and partially reverses the tax and trade penalties, so that if you keep pouring money into it could eventually become borderline viable.

Steppe peoples will also face a challenge when expanding out of the steppes, as units recruited outside the steppes will still have the full recruitment and upkeep costs.

I'm not sure if it can be done, but if steamrolling with numerous and cheap horse archers of doom is a concern, maybe the steppe units can be horde units, so that they disband upon taking a settlement? This way anyone who ventures into the steppes will die horrible, horrible deaths, but those same orange stacks defending the steppes will not penetrate further and much quicker than they historically did. iirc Invasio Barbarorum had a similar approach with the Huns.

Just some thoughts.

[Num13ers]

The North-East is already a to conquer. No need to make it even harder.