We've been internally playtesting various iterations of these rules for about two years. We are going to be continuing to playtest between now and the release (my lunch-time Temple of Elemental Evil game met today, where they are rebuilding their characters for the third time). That said, while this has taught us a lot about the game, it's not the scope or size of the playtest we want. That is why we are doing the open playtest. That is why we have a large, multi-chapter playtest adventure that that tests certain aspects of the game in this broader scope. We know that this level of playtesting finds things that smaller internal playtesting does not. That's one of the chief reasons why we are doing it.
Which is why I would have liked to see something closer to an eight month playtest rather than a four month playtest.
For most groups, four months is likely 4-16 sessions depending on monthly or weekly play. An average is likely 8 sessions, bi-weekly. Probably 7 as one inevitably gets missed. That's enough to see three levels or play. Maybe four.
Fully testing a full game of twenty levels with a dozen classes just isn't going to happen. It's going to result in a whole lot of low level play and increasingly less play at high levels. And the rare groups that do try and test high level play are unlikely to have a perfect grasp of their characters enough to spot the abuses and imbalances.
I'm not really asking for myself.
While getting the PDF early would be cool, I know my group would be unable to actually test. I play in two games: a Star Trek Adventures game, and a 5e game where I've committed to running
Tomb of Annihilation and we won't be finished by the playtest window.
Carrion Crown is burning a hole in my gaming shelf and that'd be fun to upgrade and test that AP, but the timing won't work. I even considered a third game, but I think my wife would divorce me.
But I'm sure there's a way other people (i.e. not me) could have been giving early access.
Maybe subscribers to the Pathfinder RPG line. The Adventure Path Charter Subscribers for sure. People who wouldn't be buying the book from stores anyway. Paizo has a history of getting PDFs to subscribers slightly early: this is just a couple months early rather than a couple weeks. Five-star PFS gamemasters would be another good group.