Agents are not incentivised towards keeping listings fresh. It's better for them if they can tell the person that gets in contact that it's "just gone" and hem-and-haw about how the market is very hot at the moment. They can then show you a selection of over-priced, not as attractive properties and try to extract a purchase from a slightly more anxious version of you.
I'm not sure if this is solvable with an app to be honest. And, you will be competing against listings with attractive-looking properties that aren't actually on the market, so on the surface-level won't look as good. Good luck!
I think a lot of the issues would be solved by moving the power from the agents and lettings management services to the tenants and landlords. OpenRent is already trying to do this, and I've had good experiences with them so far.