First of all, I would recommend departing at 8 o'clock because it will presumably be less crowded than at 10 o'clock.

presumably 是由动词 presume（假定、推测）加上副词后缀 -ably 构成的副词。presume 本身由前缀 pre-（预先）和词根 sume（拿、取）组成，字面意思是"预先采取某种看法"。presumably 表示"据推测""很可能"，用于表达基于现有信息的合理推断。