I don't this so, as this is the chance of 1(single) key. 🤔
If you were to reverse a PubKey to PrivKey, then indeed there is a reductionfdfrom 2^256 to 2^128 and that because Birthday Paradox(Misnomer)
You are also correct regarding collisions with addresses.
When calculating the Bitcoin address you at some point do RIPEMD160 and you can find colissions there.
This project focuses on this https://lbc.cryptoguru.org/about
The probability of collision is 100%(because of the pidgeon hole principle:p)
I think the question is how many steps it takes to find one, and the answer is sqrt(N).