I tried the game and its not bad however, ill tell you from experience since i released a game of the exact same genre like a week ago and a bit and ran ads
your game needs more kits and i mean alot alot more kits over 15 and make the progression ramp up from fast to super slow thats a well known game design thing though already
as for the main subject the retention, I recommend adding bots, its something I wish I did before moving next you can be smart about how you do it but its up to you,
also i would focus more on your lobby and making the game more addicting and also when you jump you glitch into the floor for a second