Generally, tall riders do better in forward flaps (think XC type saddles) or saddles with further stirrup bar placement. It depends on where your height comes from. I am not abundantly tall but I have very long femurs. I’ve felt my best in a Black County Vinici and a Stubben Zaria; both have a forward flap that accommodates tall riders well.
Mono versus dual flap should not affect too much other than rider’s feeling of close contact. Some monoflaps have important billet systems that are an improvement over older style 3-billet systems, some are adjustable, some are not. The billet alignment in a monoflap can sometimes not jive with the barrel shape and girth groove of a horse, so always something to keep in mind. Having ridden in both I’d say I prefer monoflaps for their closer contact feel, but I do have a few standard flaps I like.
Knee blocks really depend on rider anatomy, can’t say too much there other than try around.
I recently went dressage saddle shopping for my upcoming young mare. I tried a lot of saddles and truly felt like it was a Goldilocks Endeavor. This one is too deep in the seat! This one’s thigh blocks are massive! This one perches me forward! I learned I don’t tolerate a saddle that holds you in with deep seats and a lot of blocks; which is disappointing, because it seems that’s all the nice saddle brands are making these days.
All of that is moot if your horse doesn’t agree with the fit, which is a whole other layer to the mystifying and sometimes frustrating journey of saddle fitting.
You’ll have to play around. Do you have any fitters local you can call and explain your dilemma to who would be willing to lend you saddles to try? If you share your area I’m sure COTH can help with recommendations.