Over 3 yrs ago we replaced most of our replication with a readable AG secondary replica. We have never regretted this. Replication worked ok, but was much more problematic.

To do this you don't need any FCIs at all, just need to have a "cluster" (WSFC) that encompasses all three nodes A, B and C.

If you need to have high availability then easy ways to address this include FCIs or synchronous AGs. You did not mention HA, so seems to me maybe this is not a requirement? If you decide it is, I think you could use nodes A and B to provide HA for each other, and leave C as a reporting-only async readable AG replica. Absolutely cannot overlap an FCI with your readable replica containing same data, so no way you can have either FCI extended to node C.

PS -I think getting from where you are now to where you want to be with minimal downtime will require some thought/planning. This is not hard at all if you have 3 more boxes/hosts that you can just migrate into. Otherwise the AG "no overlap" rule makes the resulting shell game somewhat challenging.