Solo Deep Dungeons are akin to Ultimates, in my eyes at least, challenging content that requires substantial trial and error, and are hardly for everyone, but the people who enjoy them adore them, personally, I think group DDs are substantially more fun, and you can bring any combination you want, want 4 tanks? 4 Healers? standard Light Party 1T/1R/1RDPS/1MDPS? 4 DPS? you can do whatever you want, and it's all viable even if non-standard party comps feel incredibly shitposty, I even would legitimately advise people to try reaching the end of each Deep Dungeon with a full party, so long as you can find people who can reliably play to get there, but I will hardly fault anyone for scoffing at the idea of doing any of that nonsense solo, and if you think someone is fine for not wanting to do Ultimates, the same apply here.