I love the design of this Civ, but I must say the yields on the Royal Kurgan are absurdly overpowered. It could be a much worse building (or not exist at all, even) and still, due to the strength of the rest of the Civ's abilities, the Civ would easily be top tier. Personally I'd recommend toning down this unique building substantially, just so it doesn't feel so 'cheaty' to play as this Civ. At the very least I think limiting it to just a Culture adjacency makes a lot of sense conceptually and balance-wise.
One thing I did notice, too, is using the TCS Tooltip Mod causes the "Horse Pasture" to show up as an "Improved By" option for Horses when playing with every Civ, not just Scythia, and that mod is not formatted appropriately to account for this new Improvement. It does not do this for any other unique improvements, even those from becoming a Suzerain while those improvements are unlocked, so maybe there is some way you can find to have the TCS mod ignore the improvement as an option for compatibility's sake.