Once the Assassins eat all the snails they will need a food source. They can also reproduce too. Also buying loaches is stupid. Most things won't eat malaysian trumnpet snails. Puffers won't even eat them. Also assassins go for weaker/slower snails. So if your pond snails are fast and healthy they won't get them.
@ Isellcars, Assassin snails are not selective, they will attack any snails they can aside from MTS.
Best thing to do about snails is:
1. Learn to like them, they are really not that bad and they can be beneficial like cleaning your tank, breaking down waste, provide natural food for your fish.
2. Feed less and clean more
3. Set traps like bottles in the tank with food overnight and in the morning there will be a good amount in the bottle. Then just discard.
It is usually more hassle to try and get rid of them, and more often you can't get them all unless you tear down the whole tank. If you wanna waste your time and money and add more bioload like buying loaches, chemicals, etc then go ahead. But IMO it is better off learning to live with them.