I recommend going off a big jump where you will have at least a couple seconds of air time as this will make it easier to pause.

Make sure you pause while your character is still airborne.
As long as the game is paused, it will continue to count the time spent paused as time airborne. You can check how much time you have spent airborne by looking under the Me tab > Statistics > Total Air Time.
Based on how much air time you currently have and how much you need to get the achievements, you can decide how long you want to leave the game paused for.
I don't recommend trying to do it for over 20 minutes at a time as I ran into some issues unpausing and you have a higher chance of losing your progress since it will only award the time after you unpause.

After spending the desired amount of time paused, unpause the game and stick the landing. At this point the game will credit you for the time you spent in air and award any associated achievements.

The same strategy applies. You need to find somewhere to grind, pause while grinding, then unpause once you hit the time you need.

Shout out to shhh, cutting my thighs for coming up with this strategy. See their guide for a video. https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=659414148

Also shoutout to Revoltyx’s guide for enabling me to play the game again!
https://steamhost.cn/steamcommunity_com/app/244930/discussions/0/3940146862834581970