As is with all games, a balanced all-rounder basically sucks at everything compared to the more specialized Servants. The only way an all-rounder can actually shine above the rest is if they have values that're akin to specialized units. And if they do, they're generally so busted that they outclass everyone that they're compared to. Imagine Constantine being compared as "QSH except he can protect the party as well" or "regular Mashu except he can do a buttload more damage than Mashu in Orteanus outfit can". You can see how bad the entire situation could become. All-rounders will never be able to make it out of "this unit is bad"-land without drastically overtuning them.
Now, does that mean he shouldn't get a buff? Not really. I'd like to see him get 20~30% Atk Up instead and his Damage Cut duration be changed to 3-hit, 3-turns instead. It won't make him busted by definition but it changes how you play him and how you want to get the most out of him while giving him a slight bit more QoL. The Atk Up boost means that he'll provide way more value than most other Charisma+ Servants whilst the Damage Cut change means that you could use his 1st skill after his NP and still get away with it rather than take the all the damage from that turn. Obviously, the latter is more important for performance compared to the former and if you managed to "loop" his NP each turn, you get a huge payout for all the Roman Servants on board.
I think... him + double Koyan of Light can already do an NP loop three times in a row? If you're lucky. Anyways, it's not the best scenario since you're only tanking the first three waves but it shows that he *can* perform.
Probably just displaying him repeating the "Fall of Constantinople" with all the Romans he's trying to save.