Prop Firm Payouts and Brokerage Backend

Curious if anyone's had deep dives into the actual broker backend most of these prop firms use for payouts. I'm talking beyond the flashy websites. I've been comparing a few, and the spread differentials on major pairs like $EURUSD, especially during busy hours, seem to suggest wildly different liquidity providers. Are we seeing firms choose cheaper, less liquid partners to maximize their cut, then passing that slippage onto us? Seems like a hidden fee. Or is it just varying levels of sophistication in their own execution tech? Also, how much friction are you all experiencing with KYC/KYB for larger payouts? It feels like some firms make it deliberately opaque once you hit a certain threshold.