Storing scripts (smart contracts) outside the blockchain
First, yes you are correct in that each time you spend from the same p2sh address, you're specifying the script that hashes to that address. And yes, if you spend multiple times from that same address, you would be correct, this would be highly inefficient since you'd have to reveal the script each time and the script could be very long.

However, in bitcoin, it's not considered safe to re-use addresses. Each p2sh address should be used once, not many times like in your scenario above. In that case, it's not inefficient or suboptimal at all. That said, I suppose something like segwit could be used to store scripts off-band, but that would be a significant and complicated change for not much gain.