The game already has a lot of choices, but they are too shallow and "modular". Like here You already have a special unit thats tied to absolutism mechanic, its the cossacks. More cossack units, less max absolutism. Its pretty "meh", the problem with shallow choices like that is that you can easily look up under the hood and calculate what is the meta and suddenly "CHOICES" became optimal and suboptimal choices across the board. Oh and of cource the mechanics cant be that important to the game, they must be modular, because they are available only to the players with DLCs.
EU4 has plenty of stuff that reward you for having low absolutism in the form of priviliges, but its shallow, meaningless and it gets drowned in million other random green numbers (oh, you have folwarks? This incredibly important economic institution? That would be +10 in...*thows a dart*...production efficiency!). Just adding buffs without thinking damages the game more than it helps. Your suggestion is not bad, its on the right track but difference between absolutist rule and oligarchic one is too important to be summed up by a single number that isnt even properly correlated with the influence of the estates. And the declining influence of heavy cavalry or efficiency of cavalry in different geography is not very pronounced either. So you would end up with a choice that weights admin eff against Polish special unit, that will most likely have a meta applicable to 95% cases.
If you have choices, it is important to look at why did some states chose certain routes and why did some states chose the other route. Then make that choice significant with good and bad things for both directions, with a reasonable incentives to choose one or the other depending on your situation.
Why did Poland choose the route of "golden freedom" IRL and why did it eventually failed in competition against absolutist regimes in (P)Russia? Here the answer can be relatively simple. Nobility made up the best heavy cavalry in this period, but there were social problems associated with strong nobility; the nobles were not fully obedient to the crown and thus the state faced internal competition. Heavy cavalry progressively lost its importance on the battlefield with better and better firearms. On the plains of eastern Europe heavy cavalry remained the best fighting force much longer, to the late 17th century. Prussia and Russia on the other hand were more into absolutism and subjugated their respective nobility, so while they couldnt match the melee prowess of Polish nobles, they formed stronger societies that eventually prevailed. Of course things were much more complex in reality, because Poles suffered from Swedish deluge, but most of their problems (deluge largely included) can be tracked to the nobility acting in their individual interests instead of collective interest of the state. Adopting an absolutist rule of course is not without problems, nobility is actively working against you, you will lose the advantages that made aristocrats a thing in the first place; but its an investment that pays off in the late game. And this story applies only to continental Europe, Britain has its own story why parliamentarism won against absolutism.
BTW there are already very good mechanics for modelling low/high absolutism in EU4, its the MEIOU and Taxes mod. Paradox can easily just adopt those into the basegame.