Keep Zin, try to offload Mendy.
Walker isn't getting any younger but he has a big contract and necessitates games on a regular basis, certainly when Pep wants to rest or rotate Cancelo.
I don't see the need to buy a left back when Cancelo also looks great there and can be kept further sweet for games, covering Zin for rotation or injury, especially as Ake and Laporte can also shift across in emergencies.
Added to that, City could bring Yan Couto back this summer and have big hopes for him at right back.
That will allow all of the above and ensure Couto is ready to get games as the potential Walker successor, back up to Cancelo on the right.