Regarding a progression board i was/am in the same situation like you, learned on an 7'11, 130 liter, 4in1 board (Mojo custom, shape like F-One Papenoo) and now changed to a Mojo Custom 5'8, 105 liters. I'm 93 kg nacked and i think the rule +10 liter above bodyweight is quite spot on if you use the board on gusty lightwind sweetwater spots like i do.
After some sessions I think i could have gone to 5'6 - 5'7 and 100 - 103 liters, but below that slogging and standing up in wind holes get's difficult (for me).
Regarding the shape i like the clean shaped boards better, reduced bevels and kicktail and hard edges, like the Fanatic 5'4 or the Indiana boards. My board pumps and releases really well and I never got stuck in touchdowns with my 5'8 so i don't see the advantages of heavily beveled boards like the Quatro Wing Drifter. Also i think extensive bevels and kicktails do make a board unstable when kneeling and starting by reducing area and rail volume (and may lead to demand a bigger board for equal stability to a "clean" shape)
If you're Europe based and want to go custom, maybe you like to take a look at the Mojo Customs from Hungary, they have a new T-Bird Wingboard line, whereas their routes lie in building windsurf boards, like Flikka.
Regarding availability Mojo needed 7 weeks from ordering to delivery (Hungary to Germany).
Since they do CAD shaping you can order any size and volume from them.
Weightwise a 105 in light carbon sandwich (not the lightest but tough windsurf technology) comes with 6,8 kg, mine has an undestroyable Ultra HD layup and weighs 7,6 kg, 30 cm long foilboxes included, so much room for positioning your foil.
For you a 85 liter board board should come in around 6 kg.