So a couple big weekends that are crowded everywhere here! You repeated that! Yes there may be some extra lines for Songkran ( and quite frankly prob not so much for koh chang ferry) that ferry leaves very regularly and is so cheap and easy to bring your car! No bridge will be or should be built. You are way off and constantly comparing one or two irrelevant weekends of the year where it's busier. If most people here remember before Covid sooo many places like koh chang were pretty quiet even during high season. I was there 2016, 2017, 2018 and 2019. It's quiet. The ferry takes cars! There will be a bridge to koh samet before koh chang.
BTW I've had less than 10 min waits 3x!