LSA and Microlight are going to be quite different in your context of STOL behaviour.
Microlights are up to 35 knots stall speed, and depending upon country somewhere in the bracket 450-495kg.
LSA (Light Sports Aircraft) also vary a bit, but will nominally be up to 45knots stall speed and 600kg.
I think that if I was running something like this I'd go for some nice round numbers...
"Microlight" - MTOW below 501kg, stall speed not greater than 35kts CAS.
"Very Light" - MTOW below 751kg, stall speed not greater than 45 kts CAS. This will pick up the LSA and VLA categories and a lot of nice smaller light aeroplanes.
"Light" - MTOW below 2001kg, stall speed now greater than 61 kts CAS.
That would align quite well with present regulatory categories, but give the flexibility for people to shoehorn aeroplanes in and game it interestingly.
I should ignore Annex II (now known as Annex I) - all that defines is aeroplanes not managed by EASA.
"Experimental" means different things in different countries. But, at first stab, you could divide categories between "as certified in state of registry" and "modified". That would be relatively honest, but good luck getting permission to fly all these foreign modified aeroplanes.
If you want to create some possible interest and excitement, try adding a category for "fully electric".
G