On the drop rate for keymolds, I'm pretty confident that it's actually 1/2500. This is what the console version was when it got the 1.2 update and the mobile version should be identical to that one (minus the more recent frost moon updates).
I had a farm for rainbow slimes that wasn't much more than a simple death pit with lava and dart traps as well as a couple small blocks above it so the unicorns would hit their head and fall down. I had a tunnel to get underneath using actuators on the blocks, which led to your typical "survival room" (honey, heart lantern, water candle, etc.) Using a series of defensive items, turtle armor, and making extra sure that everything was filled in below me, I was able to get tons of rainbow bricks and keymolds. It wouldn't take much to change the biome to something else.
That being said, it will take time. Even a proper afk farm can still take several hours to produce a keymold. You definitely don't want to use a farm and then check back every 5 minutes; that will drive you insane. My suggestion is to either find something else to work on in the game and just let them (hopefully) come naturally, or make an afk farm and watch a movie/show before checking it again.