Gentlemen.

What material lengths, and thickness of flock you are recommend for spinning rod grops?
Polyamide, 1/50” length, 3.3dtex ?

I think, that a flock - not most suitable variant for furnish of rod grips.
Flock is pluck-out, get dusty and reduces sensitivity.

Probably, good photos of flocking grips will change my opinion.

PS
I’m in flock industry about 13 years and I know about flock, adhesives for flocking and high-voltage electrostatic equipment a little.

Nylon, .05 or .10 inch lengths. Flocking has been in use on fishing rods for roughly 50 years. It is featured on some of the very most expensive and fine quality fishing rods ever built. It holds up very well. There is no reason why it would reduce sensitivity and it can be cleaned/washed as often as you like.

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I have checked all my flocked grips and so far they all appear to be fully valid.

Seriously, whether or not a grip gets dusty depends on whether or not you keep it in a dusty place. My are holding up well . I have an older boat rod made back in the early 1970's. It has a flocked grip and has seen a great deal of use. It is still going strong. I wish I could say as much for the rod blank and guide wraps.