I came to believe that an ISP can easily detect full nodes, BTC connections and isolate them from reaching any other nodes.
When and if an ISP decided to do so, could they easily perform a 51% attack and do everything they want?
Yes, but this would only work if said ISP would host more than 50% of all the full nodes in the network. This is simply not the case, the nodes in the network are too widely distributed for that.
I came to believe that an ISP can easily detect full nodes, BTC connections and isolate them from reaching any other nodes.
This is true. An ISP could easily block all Bitcoin traffic or, just prevent Bitcoin traffic from entering/exiting their network and isolate the nodes.
When and if an ISP decided to do so, they could easily perform a 51% attack and do everything they want?
In the case of isolated nodes, said ISP would still need enough hashing power to present blocks with sufficient PoW to be accepted (a LOT!) and, would have to also block Tor.