Buying a current agency already set-up and online is an excellent starting point, IMO.
You don't need your own agency. If contributors were able to act in a unified manner, then it would probably be no problem to agree with the most reasonable existing agency on some form of exclusivity.
The least reasonable agencies would be boycotted, leading to their demise, and everyone else would have no choice but to negotiate and give us a higher cut.
How much % are we really getting now? With unused subscription payments? 5-10% ?
Imagine the real value of our work if someone was paying us 50%.
But that should have happened 10-15 years ago, before microstock became a mob thing. It's too late now. With Ai, there's no chance at all.