This is my read of the situation.
The players seemed, according to the description, to be having fun.
The DM seemed to want the fight to be more challenging. By more challenging, I mean have a greater chance of using up resources, causing permanent damage to the PCs, and/or possibly threaten character death. Because the fight wasn't overly challenging, the DM had less fun.
It seems to me the simplest way to have the DM have more fun is simply to increase the potency of the challenges.
There seems to be an undercurrent of expectation that running the core rules in a standard adventure should provide a baseline higher challenge than the rules currently provide. I feel that expectation is controversial, but whether or not that expectation exists, the die is already cast in terms of what the rules actually provide.
As a solution, the DM could simply change their expectations about challenge level, and enjoy giving the players the challenge they seem to prefer. Or, they could change 5e to be more challenging. This can be done by changing player-facing rules, or by creating encounters that are significantly more deadly than the "standard" or pre-published adventures. Or, they could play another game where the default difficulty level is closer to the DM's expectations.