I've played the 4 Pawns for a long time, and I think it's badly underrated.  Sacrificing time in exchange for space seems to be increasingly consistent with modern practice; here and elsewhere it produces very dynamic situations.

9...Bg4 followed by ...Nbd7 went from being THE counter to White's idea (in the early 90s) to a backwater, with scant explanation from its former advocates.  It seems, however, that White's early Re1 turned out to be too much to handle.

I haven't considered 9...Nbd7 much, but offhand, it looks a little passive.  Relatedly, after 9...Re8  10. e5 dxe5  11. fxe5 Ng4  12. 0-0 Nxe5  13. Bf4, ...Nbd7 either now or after the exchange on f3 is strongly met by d6.  However after 9...Nbd7, Black's rook still defends f7, so I suspect 10. e5 immediately may be premature.

Even so, I think that e4-e5 should be White's goal once Black's e-pawn has been exchanged for White's c-pawn.

Black is under some pressure after 9. cxd5.  E.g. definitely Black should not play 9...a6, which is strongly met by 10. e5 dxe5  11. fxe5 Ng4  12. e6 fxe6  13. Ng5.

This is my feeling also - although it is quite usual in the Benoni to play ...Bc8-g4xf3 for the reasons I have indicated, I sometimes feel my light squares can get a bit 'airy' afterwards, especially if White starts lunging with his e- and f-pawns.

Well, I am probably more of a mere mortal than you - at least until I hit the 500 posts mark and join the deities on Mount ChessPub

I have been playing this with Black (via KID and Benoni move orders) for years.  I think it probably suffers by comparison with 9...Bg4 not because it is objectively any worse (indeed, it may even be better), but simply because 9...Bg4 is more natural and feels 'safer' (reducing White control over e5 and reducing cramp in the Black position).