I think that the Kode 86 would make a lot of sense
- It is 20 liters smaller than you K105, a nice difference
- It can take a 5.5 well and slightly bigger comfortably (remember that smaller persons can use bigger sails on a given board!)
- It will not sink you as brutally as a 80 liter or smaller board. In the real world this means that you will get more surfing days on a much smaller board = Fun!
- You should be able to use your 5.6 for both K105 and K86, and switch during the day.
- When it is blowing 4.0 for a 90 kg guy there is quite a lot of wind... Quite a lot more (like >10 additional knots) than you can manage on your current gear.