That's understandable, didn't consider the alt characters at all. And I guess making the quest starter like loyalti button just for every 3 day is not really a solution either (not really familiar with the limitation of that).@Updown spawning boss every hour is not a good idea as it's not just against the event story but will also not reduce the number of players. In that case players will just use multiple characters on the account to visit the event.
And I am glad to hear that you take this experience and try to improve it next time. This event was not a bad idea (however, a little bit less on the rng part would be welcome, reward ranging between 20 gold to 10k+ gold), and I haven't seen any MMO which could handle 200+ players well. That's why Blizzard haven't really released a big server event since the Ahn'Quiraj opening (on the other hand that was 10+ years ago).