More HP doesn’t make something harder……….unless said HP makes it so you can’t skip the bosses most dangerous mechanics
For example the first boss of dun scaith using your example, his two most dangerous moves are void aero 4 and his soft enrage. In an average fight these days you’ll see void aero 4 once (at launch it wasn’t uncommon to see it 4 times) and his soft enrage never comes into play because you will only ever see him fill maybe 4 of the 8 tiles with the void flooring (and you never see his most devastating combo of void aero 4 into soft enrage with 7 out of 8 tiles in danger)
Orbonne is still 40+ minutes even with echo so that basically just proves my point
Alliance bosses are given a specific amount of HP because it allows them to show off their full suite of mechanics before they die, bloating the ilvl synch or reducing the actual boss HP (like in myths) means you skip the bosses actual difficult combos of mechanics which makes the fight way easier. If you cut red girls HP by 15% you’d no longer see double meteor. Cut 3rd puppets HP by 10% and you’d no longer see the wall bot mechanic with only one tiny safe zone, cut 9S HP by 10% and you’d no longer get swoops+red floor AOE’s