There is an elephant in the room with the new estate system: Special Unit Build Limit.
Some estates, like the Cossacks, will allow you to build special regiments depending on their land share, and here in lies the issue; it is incredibly difficult to field any meaningful amount of special regiments that are tied to land share because it is exceedingly hard to give land to any one estate without all of them also getting land. Moreover, the scaling special regiments share is really small and will necessitate that you have a very large FL before you can field more than a handful. To make matters worse, some missions require you to have at least X regiments and these missions weren't updates to take into account how hard it is to recruit them now, meaning that these missions are exceedingly difficult.
I recall having this issue with Mewar and the 20 Rajput regiments achievement. However (and completely by accident), I noticed a few days ago that, when you grant a privilege that lowers your crown land, all that crown land actually goes to that estate (instead of being distributed, e.g., when selling titles). I imagine that you can actually control it quite well, with the proper management. (You can also revoke a privilege and grant it again to grant that estate more land this way.)
For example, Mewar starts with 20% Rajput land. Establishing the Rajput regiments raises that to 25%, and a 12.5% special unit force limit. Granting Primacy of the Rajputs and Rajput Land Rights (or Enlist Purbias), and then revoking and regranting Primacy nets you 50% Rajput land share (25% special unit force limit) before unpausing (although it also lowers you to 19% crown land, but you get the idea

). Seize land to revoke land shares from all estates, then every once in a while revoke and regrant a privilege to the Rajputs to increase their land share; I suppose you could (eventually) get them to a 70% land share that way with the other 30% being crown land, or anything up to 100% if you don't mind the penalties.
This requires you to be careful about granting crown land privileges to other estates, but even if you do so, you can still reach those numbers. It'll just take more cycles of revoking land and refreshing privileges.
As for the Cossacks, they lack a 10% crown share privilege but they still have four 5% privileges to play with. It should be possible (in time) to field anywhere up to 50% of your force limit as cossack regiments (or up to 35% without suffering crown land penalties).
Edit: make that up to 100% of your force limit; I missed the second privilege doubling your cossacks force limit after you grant the first. Shame on me for never using them, I suppose
Mind you, I haven't actually played with this yet, so I don't know if this approach is practical

I'll have to revisit Mewar (or something in the area) sometime soon to try it
