The main issues of stopping or mostly stopping WCs from the snails' standpoint would be be gradual CaCO3 and other trace element depletion that would lead to erosion of the old shell and/or growing inferior quality new shell. In the past I've done smaller snail tanks with plants (albeit very basic plants like java fern) where I primarily did top-ups and added mineral replenishments in the new water and only very rarely did a WC for cleaning purposes if I saw some kind of debris buildup that needed to be siphoned out, or if I badly distrubed the substrate somehow. Even done that for small sw "planted" invert setups with macroalgae. That's how I will be running my recently set up Planorbid and java moss bowl too - mostly doing WCs when I disturb the tank by trimming the moss and otherwise just doing top-ups with mineral replenishment. I can't speak to any of those things though from the standpoint of plants that are fancier than java fern/moss or what would happen if there was something like a CO2 system involved, since I've never used those.
I even toss an occasional single shrimp pellet into my Planorbid bowl just to make sure they don't start chewing on each other for protein. They typically mob it within minutes, which suggests they need it. In a more complex ecosystem, that kind of thing is less of an issue. If the snails in this case are eating the flake in a timely way and not just letting it decay, then they're getting something from that and may not necessarily find the same nutrition elsewhere in the tank.
I tend to prefer shrimp pellets as an occasional food for snails since they contain some things that are good for shell formation and they sink which can make them more accessible, but if the snails eating flake well and the new shell growth looks good, why fix what ain't broke.
