I never played the DMM version and I have no idea how the later content will go, but I play multiple accounts in this version and I've rerolled and played through levels 1-30 enough times to be a Union all by myself, all with different team setups. As such, I feel I can speak fairly confidently about what's required with what we have at the moment.

For the present game and the Event that just passed, literally any garbage team with enough health not to get 1-shot and a damage source will do. My first two accounts, 1 with a (Prereg) Beelzebub, Artemis, and Brynhildr and the other a reroll with Dark Amon and Tyr, could both easily do the Expert Event with no problems whatsoever at about 14k power. Neither have any real elemental focus (in terms of weapon grid farming) or a lot of thought put into them, they're just the first two accounts I had that got a decent roll so I leveled them up. They can also both solo Standard and Expert raids, although it takes a while because Expert has so much HP. It's really just me sitting there with autoattack on and waiting for cooldowns though, there's nothing "dangerous" about them at all.

In the teams I've rerolled since then I mostly just do the level 1 to 30 run to see if I get something good on the second 3000 jewel gacha and the SR ticket, but they're not fancy either. Usually just two SRs of the element I wanted to experiment with or an SSR+SR if I'm lucky. Again, within 48 hours and no time to farm up a weapon grid or anything special, they can get to 13k and be good enough to solo a Standard raid or get "babysat" through an Expert raid if I open it with one of my stronger accounts.

If any trash team can solo Expert Raids and runs within 3-4 days and literally no weapon grid farming, I can't see how (aside from efficiency of course) it's completely detrimental to pick whatever you want. You could of course be doing the content *faster*, but seeing as the stamina regenerates so slowly that isn't really the worst problem to have.