WI a king, with stronger position and bigger power, assured better treatment of the Cossacks? IIRC at the beginning of Khmelnitsky's Uprising the Cossacks claimed to fight the noblemen, not the king, whom they considered their protector and defender of their rights. WI fairly treated Cossacks remain loyal and ready to fight for PLC against Tatars, Russians and Ottomans? Polish-Lithuanian armies together with Cossacks would be a power to reckon with.
Actually for me that is astonishing how long the Cossacks stayed loyal and manageable in OTL. Any longer would be pushing it, I mean in ATL.
I mean, the Cossacks are Orthodox like the majority of population in Ukraine and Belorussia.
And the PLC king is Catholic like overwhelming majority of the political elite of the PLC. Whatever you do the Cossacks would feel mistreated, underappreciated and "second grade".
That's splitting the PLC.
And having these Orthodox people armed and organized...
That's not the brightest idea,
it's like for the British Empire to have the Irish Army in Ireland consisting only of Catholics from down to top - all the soldiers are Catholics, all the sergeants are Catholics, all the officers are Catholics and the general is an Irish Catholic.