So I had 2 apple snails that kept laying bright pink eggs, and we kept removing them, then we found a little snail. We assumed we had missed an egg and welcomed him/her to the family. They kinda stopped growing and since then lots of little snails have appeared, but no pink eggs have been laid to remove.
I am now thinking baby apple snail may actually be a mummy pest snail.
My plants are slowly being destroyed and the snail numbers are growing.
Have just spotted what looks like a white egg cluster?
Best way to deal with my snail issue and not kill off my apple snails, shrimp & fish?

This is the biggest of the little snails, so thinking deffo not apple snail

To avoid harming the apple snails I think your only safe option is manual removal of the pests.

I wondered about Assassin snails. Do they take out bigger snails like apple snails? And do they harm shrimp?

Yes, assassins would attack apple snails as well as the smaller ones.