So, I did this on the Dragon Prince wiki too, and thought it was fun, so here you are: my headcanons on what characters in Tolkien's works might be autistic and things like their stims and special interests.
Legolas: Stims might be shifting from foot to foot, blinking rapidly, or rocking back and forth. I think his special interest is definitely either trees or elvish history.
Sam: Stims might be clapping hands or biting his lip, safe food is potatoes (PO-TA-TOES) and his special interest is types of flowers.
Finrod: Stims could be clicking fingers or fiddling with small rocks/hair ornaments etc, his special interest is dwarvish culture (he's also the kind of autistic person that is hyper-empathetic)
Faramir: Stims are humming or tapping in time with a rhythm (even if there's no music playing) and safe food is bread because bread is good :) Idk about special interests though, any ideas?
(Just to make it clear I am also autistic I'm not just assuming I know what I'm talking about)
Tell me if there's anyone I should add!