Think in “last appointment ends”, not closing time
If you want to be done by 6, your last 90-minute service starts at 4:30. Setting hours as “last appointment ends at 6” keeps long services from running late.
Buffers are part of the appointment
Cleaners and detailers need travel time; lash and nail techs need setup and sanitizing. Add it as a buffer so it's never offered to the next client.
Evening and weekend schedules are fine
Many independent pros work non-standard hours. What matters is that they're consistent enough that your replies — and any tool you use — offer only times you actually want.