I can’t vouch for conditions on any given day, but when we were there over Thanksgiving I never really saw long lines (or any lines for that matter) at the buses that early - and I was often around them during that time either out running or using them myself. Between the boat and bus I think it spreads the crowds out reasonably well. 90 feels safe but overkill, 60 -70 is probably OK.
But don’t come back and yell at me if you encounter horrific lines if you get there 60 mins early and it turns out I have given your terrible advice with how things are at the moment!
Let us know how it goes - we can never hear too many first bus of the day reports.
Enjoy your stay!