The Veerse meer is a perfect spot for the Ultrasonic.
Why ? because it is full of lulls (depending on the wind direction).
It's one of the most beautiful lakes in NL, I find.
Windsurfing with the charming town Veere on the background, is simply idyllic.
The Ultrasonic will handle the lulls much better and point much better than the Ray145, because of its bigger width and fin length, used on it.
Is it better controllable in the chop c/w the Ray145 ? There won't be much in it.
If it gets uncontrollable, you should be anyway on your Fu111, certainly with your 75kg.
Until now, I only had 1 serious tailwalk with the US, but it was in a very strong gust and probably my own stupid fault not having sheeted in enough.
Important to know is that the board feels VERY light. I never have the feeling that I am on such a wide board. Probably the tail cutouts will also provide better control c/w the Ray and will largely compensate for the bigger width.
On the Grevelingendam, where the chop is always nastier c/w the Veerse Meer, I've never had any problems with the US. I'm a bit heavier than you, of course (87kg).
8.8 is my smallest sail on the US. I often start with 8.8 on my IS122 and when the wind gets too unstable I just swap to the US.
In very light conditions I use a 10.9 on the board. Feels also great.
Also read the following thread, where many have posted their experiences.