Regarding maxing-out monsters. I hit a pretty hard wall in the normal dungeons with my ADK team, once I hit Vesta Cave and beyond. My monsters levels were around Lvl.70 and it's the first time in the normal dungeons that I had to switch strategies and slowly beat most levels with Odin and various resist monsters. I eventually maxed out my ADK team, and I've since reverted back to it for farming those previously unbeatable dungeons during 1.5x events. Maxing out my ADKs and CDKs made a huge difference in being able to reliably kill those 600K+ HP bosses with 11K+ defense. And ADK and CDK have flat exp curves with very small gains at high levels. Yet, it was still enough to make the difference needed. The gains would be even greater with gods, since they have exponential growth curves, and really benefit from higher levels. And really, since I've started farming for +1 eggs, maxing-out monsters no longer seems like an impossible chore in comparison.
I'm obviously an IAP player, and enjoy collecting and building various teams, but if I were to play for free, I think it would be even more important to focus on maxing out only 1 or 2 teams before building other ones. In fact, for free players, I think an ultimate evolved Starter Dragon team is probably the best team for them to focus on maxing out. Just pair it up with one of the new Greeks of the same element.