I learned on a Techno II 148l and still have it

. The shape is a couple of years old but it's a good shape. It hasn't got enough nose rocker for bigger chop though IMHO. Ocassionally I blasted through chop instead of getting over it at 20kn wind on our local bay. Also due to its construction the Bic is noticeable heavier than other boards, especially if you compare it with the better versions.
On my board there are three settings for the back straps. Outboard, slightly further inboard and single in center. I strongly suggest to use the single center setting, it's much easier to get in compared to the outboard position. I only moved to outboard once I was comfortable with getting into the center strap. Also you need to be quite powered up to get into the outboard straps. I can't quite understand why you can't get into the center strap!? It's not further away from the front straps than the ouboard position.
For the front straps use the slightly more inboard setting as well at first. There is no real inboard position for the front straps as on other boards, but IMHO this position doesn't make much sense for a board this size. Only for learning at the very beginning.
The standard BIC footstraps are comfy, but the adjustement is crap. I found it impossible to make them wide. Even for my small feet they are just wide enough. If you have bigger feet than the problem could be that you can't get your feet far enough into the straps. How far can you get into the straps? Do your toes stick out completely?
As for your non-planing jibes don't forget to put some pressure on the back foot. No pressure there and the board won't pivot around. IMHO the Techno pivots and carves quite nice for its size without needing too much pressure (I'm 80kg though).
I think for higher wind speeds you need a smaller fin. Well, or a smaller board

. My BIC came with a 50cm fin which generates a lot of lift when it's windier. But up to about 20kn I was usually alright.
I think sailing on a lake with very gusty winds is not easy. When I started windsurfing I sailed quite often on a small lake with gusty winds as well. Sometimes it was quite frustrating, especially when I learned how to use a harness. The good thing with the BIC board is that even massive catapults with the mast onto the nose hardly leave a scratch

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I reckon a lighter board that accelerates quicker is nicer in gusty winds. If you want to go smaller keep in mind that uphauling or non-carving jibes will get more difficult. 135-140l is no big difference though.
Just keep on sailing and you will improve!