I'm working on adding some custom tools using commands. I would like one tool to allow the player to "fly", but after some research I've decided to try and settle for just allowing the player to freeze his y coord. I've tried tp-ing the player up just a little bit to counter the gravity, but couldn't find the right amount to keep the player at a constant level. I have also tried using effects such as levitation, which didn't work even at its lowest level, and slow falling, which seems to have no difference with an amplifier of 1 or 255. When I looked into it, I found a post made on the minecraft bug site with a response that they don't support effects achieved only by commands. I've heard about putting a barrier under the player which would be ok, but I don't know what I'd do if there is lava or water below the player.
I'm using Java Edition PC in minecraft 1.13.