At my wits end trying to detect player touching ground

So I've been working on this for a while now and what I need is accurate ground detection for a custom flight script. I've done a lot of testing and here is what I've found:
1. Client Ground detection using player.onGround() is very accurate. More so than running player.onGround() serverside

2. Server and client player position have like a 0.003+ desync. This causes the client to think it cant move down anymore and the server to think the player still isn't on the ground.
3. A script to round the player's Y on the server before checking if the player is on the ground works, but has the player's Y constantly being rounded. Which results in jittering.

Any help would be appreciated. I really don't know what to do anymore. I've spent hours on this one thing and am ready to be done with it.