When I have booked it has always been for a specific departure time. I usually get there early and usually they put you on an earlier train.
I always use the tunnel now, especially since then P&O mass sacking.
Their Twitter feed and app are pretty good for updates.
I typically book a specific time out and a flexible back.
Don’t forget the motorway sticker for Switzerland.
Get one of these for the French tolls, make the long journey easier.
https://www.emovis-tag.co.uk/