For the OG's, how do you Link your Mobile Sky account to the Steam version? It's only giving me the option to create a new account and link my Steam account to it.

[CrispyOBacon]

I hadn't played the mobile version in a few years, and glad to see this game now on PC for free, but I'm quite disappointed that there's no option to transfer any of my cosmetics, etc, to the steam version of the game. Why not?

[Meowish]

You can only do it on the mobile version.

So the normal steps will be:
[1] Download the mobile version and launch the game
[2] Select the login method and use your previous account to login
[3] While in the game, goto account setting and link your Steam account to it.
[4] Login on steam version and you should see your progress synced.

If you already started a new account on your steam version, there will be additional steps:
[1] Download the mobile version and launch the game
[2] Select "Steam" as login method and login your Steam account
[3] While in the game, goto account settings and link an unwanted account to it (email or spare google account, anything that you won't be using later), after that you should be able to unlink your Steam account from it.
[4] Logout and login using your old account on the mobile version
[5] While in the game, goto account settings and link the now freed Steam account to it
[6] Login on steam version and you should see your progress synced.

Important notes:
Do not use a Facebook account to login when linking or unlinking, it's bugged and you will not be able to unlink accounts from it.