I've always had a longboard then added a formula board and later a 135 ltr shortboard. If the wind strength is marginal for planing I usually decide to sail the longboard because I know I'll enjoy that. It sounds like that is not your reaction. Let's test that. I don't have a Serenity but I would like to have one. Would you? If you had enough money you could have any six boards and you are sure a Serenity would not be one of the six, then probably you should go for something that will plane early with your 8.5. If you think one of the six might be a Serenity, then find a way to try another longboard or hybrid first.
Thanks for the kind words,