The problem is, fundamentally, there's no good rule. AI isn't good enough to Dynamax at the right time in battle, because that depends heavily on PC team comp, NPC team comp, etc. So you either DMax(/mega/Z/etc) immediately, which risks the PC being able to easily counter, because they led with a type advantage or a sacrifice, and even if the player doesn't easily counter, it turns the battle into "beat the lead by sacrificing 3 mons, and then sleepwalk through everything else". Or they DMax last, which is easily countered by various setup against the mons that don't DMax. My no-Dmax runs usually turn into Sticky Web+dual screens against their 2nd-to-last mon, which isn't particularly interesting. It's a tough challenge, and I don't blame GF for having trouble finding a good solution for it.
Alola, with it's Totem Battles, was a welcome change. Lean into the "single boss enemy" vibe, once the player has beaten that, it's over, and just toss whatever benefits you want onto that boss. There's room for improvement, to be sure, but trying to beat that without overleveling and without Z moves was an experience.