Nowt wrong with doing courses, but the only thing that will really improve your off-road riding ability is riding off-road.
As said previously, a cheap 250 enduro or trailie will teach you so much more about bike handling and control than you will be able to achieve on the GS without doing it serious damage. No matter how talented or quick to learn you are.
Learning how to control slides and wheel lock ups involves falling off. Lots. Impact shorts and body armour are a good idea to let you get a bit braver.
To get to the point where it becomes instinctive takes time in the saddle sliding around. Learning to feel for lock up under hard braking at both ends with no abs is also a great skill and allows you to brake harder and further into corners more confidently. Practising endo’s on a trailie is good for this.
It’s all great fun on a trail bike and ultimately means when it happens on Your bigger bike in a more dangerous environment you have a much greater chance of walking away unscathed ( winter riding on a GS, hitting a patch of gravel on the road, or simply getting too carried away and losing one end of the bike).
Off-road bike handling skills translate to more confidence on the road, there’s a reason why a lot of pro racers ride or have ridden motocross and flat track.
And like everything else, it’s a case of use it or lose it. Once mastered, you need to keep sliding around and popping over obstacles to keep those skills sharp.
After 30 years it’s reasonable well embedded, it’s not after 12 months.
If you have a bit of spare cash and time, a dedicated dirt bike is a no brainier.