I have done Abingdon, Berlin, & Malta among others. Hamburg too.
Malta IS NOT a PB course, trust me. Net downhill is not the same as downhill all the way. Also, it's quite possible to get lost! Seriously, the lead car went astray last year & lost 2 of the lead group, and at the rear of the pack I was constantly having to stop and check the route.
Abingdon is not absolutely flat, just not hilly. Hamburg is about the same.
Berlin is flatter, almost entirely flat, but can be a bit warm. And crowded, though less so at your end perhaps.
I would recommend another one I have done - Eindhoven. Not only is it absolutely flat (not so much as going under a bridge) but it is not (unlike Berlin) crowded, the support is much better than Abo, and not only can you put your own drinks out there but they will hand them to you. Also later than Berlin so warm weather less likely.