HI Joerg,
Good question - for the North Sea, choppy/fairly "dense" wind, the 121 might feel a bit big, on the other hand you are 90kg, so a bit more volume might help. you mention 12-19knots, so that for sure is the 121, if you wanted to sail often over 20knots with the 7.0, then I would have said 115, which is our riders (and mine) absolute favourite - but the 115 is the ideal board for 15-30knots, ideal sails are 7/7.7, 8.6 is a push and works best really on downwind/not that great upwind - whereas the 121 will work great in 10-20knots, especially with a 8.6 and then also 7.7. I would say for that wind range, if you buy a good 7.7, you will not need the 7.0 - I use the 7.7 in up to 25knots, I'm 78kg´s, the Warp - can hold it forever. Especially as you mention you'll go wavesailing after 20knots. For 10-15knots, I use the 140 Falcon, or 130/121.