They still have loads of train reservations available, so it won't be as crowded at least - especially if you pick a less popular service: https://www.abelliogreateranglia.co.uk/dunwich-dynamo-reservation
"...limited to 6 bikes on a first come first served basis as per the Abellio Greater Anglia travel policy"
This doesn't mean a scrum at the station, you pre-book your bike place. Easiest way is to do it all by phone.
@Yukirin started
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They still have loads of train reservations available, so it won't be as crowded at least - especially if you pick a less popular service: https://www.abelliogreateranglia.co.uk/dunwich-dynamo-reservation