I mean, How to know ECDSA has collisions like that?
I don't think Pollard's Kangaroo will work against ECDSA sigs because there is a SHA512 hash of the message bytes which forms a second line of defence against brute-force.
So even if you cook up a Kangaroo iteration that takes you from R,S to the origional message, it's still hashed, so you'd have to find a different way around that.
Sure you can: R of the signature is the X coordinate of the curve point nonce*G. So you can use kangaroo to search for (R, y) and (R, -y). Then you would have the nonce k and could solve for privatekey.
And it usually is a sha256 hash for the message but I don't think ECDSA specifies a hashing algorithm so you can use whatever you want for the hash as long as the other side knows what algorithm you have been using if they want to rebuild the hash from the message.
So I think you should refresh your knowledge of ECDSA
.