I barely ever used z-moves, to the point that I nearly forgot they were a thing. Dynamaxing will probably be pretty similar since I don't usually play competitively. I do think that you bring up a good point. I'm really going to enjoy seeing how the meta shifts to counter dynamax. Whether it's sleep, evasion, or protecting, I'm sure there'll be ways to get around it. Doubles will be a bit chaotic when two Pokemon are dynamaxed and two are regular sized. Honestly, I'm 100% there for the memes of giant Pokemon, and that's enough for me once the post-game hits and I don't have to think about the gimmick any more.
A small point with protecting.. I'm currently thinking that only a Max Protect can protect against a Max Attack.. either that, or a normal one will only protect partly against the attack akin to Z Moves.