I think the best solution is still a compromise between both sides: If the game detects the tank(s) is the only one left alive in light/full party, the game immediately starts a 180s countdown to a hard enrage.

With that one simple change, if a tank wants to kill the boss when the healer goes down, they're now forced to work together with the DPS and keep them up or face a 180s hard enrage.

This means tanks can still kill bosses after the healer goes down if they work with the DPS, it also means that tanks can no longer waste everyone's time by focusing only on themselves and then soloing the boss from 80%.

Plus, since the enrage only counts down when the game detects only tanks are left alive, that means there's no pressure on the casuals either.