The boss timer mechanism is uncannily annoying and reeks of gacha games' styles. The bosses themselves have heavy rng based drops and add timer to that and you are just creating artificial time gate. Since there's no "pity" mechanism, that means one can spend days if not months trying to farm something and not get it.
I understand that removing rng is not a viable option, but instead of boss timers, they can introduce a unique currency for summoning bosses. The currency can be farmed from other activities and one can stockpile it to farm bosses later on.
Boss timer summons are free, but outside timer once can spend the currency to summon and farm the boss of their choices. Also, they can add high summoning cost for new bosses.