I use tums in my snail tank also. I have neber heard the growing too big for shell theory but i guess it is possible. I also feed snello, algae wafers and a snail mix i buy in amazon and once a week i give zucchini blanched. All snails doing great.
As far as changing substrate it is a pain but worth it i love the sand. Just empty the tank scoop out the gravel and add very well rinsed sand . put a plate in the tank and pour your water onto that so you dont disburb the sand too much. And fish can go in minutes after the prime.
You're gunna need LOTS of cleaning supplies and elbow grease and patients. You can use basically any cleaner to get them good and clean as long as you rinse until the cows come home. I would also reseal them before use. And water test to be sure it holds up. Before moving in the house.
I have a new one now. I couldn't do anything last night with the petstores being closed. It is running with the old filter on it until tomorrow, then put the fry into the new 20g long, Bonnie and Clyde in their 20g long with the horde of snails ,then the blood parrots in the 46g bow i sealed...