Solved

Promotion to Pro Tier

There are 3 ways I can think of that should work:
One would be: Everyone's looking for another league to be promoted in.

Two: You enable promotion in your current one and promote by 2 every season. To keep promotion open you'll have to do 6 placeholder accounts for the last 3 seasons to keep promotion open.

Three: One of you found a 2nd league as 1 car league, promotion enabled, and times it that at least one race happens at least 24 hours after your last one of your current league, its first season should be a 17 race standard season, but don't place too many races after your own season end. Then you need to create 8 placeholder accounts between you to place them in it, make sure to log into them every few days to repair and keep them from being kicked for inactivity and thus this 2nd league running. 
Also you enable promotion in your current league and set a generous season break in between this and next season so that the first race is again at least 24 hours after the last of this 2nd league. 
You play this season in the main league to the end as usual with your main accounts, 2 teams from your 10 will promote into Pro. After the season ends the rest switches over into this 1 car league, you let the placeholder accounts with too many points to be beaten leave and rejoin (you need 16 teams to keep promotion open so you still need them) to reset their points and replace them with new ones make sure your real teams score the (few) remaining race(s) and hold the 8 promotion slots (first season has twice as many, 1 car has double as many as 2 car) to be promoted and switch back afterwards into your original league. If you timed everything well those teams still enter the season break before the first race and get a full car design and thus everyone starts good into the new season in Pro.

Edit: I had a nagging feeling I'm missing something and it's that rejoining teams get their points back after the following race.