Suggested

No limit on league promotions

To encourage teams to stay in leagues it would be a good idea to not restrict promotions between divisions.
It is really disappointing to win a division and not get promoted usually ending in the player leaving the league.

I'd suggest alternatively, it'd be nice if the promotions were more staggered, particularly after the first season. For example in a 1 car league, having 1 promotion spot for 10-15 cars, 2 for 16-20, 3 for 20-24 and 4 for 25+. For 2 car it could be 1 for 6-11 and 2 for 12+. 

IDK how well this would work in practice, but in theory it should allow for Pro to build up before teams get promoted to Elite. Under the current system Pro becomes sort of the middle of a wheel pushing players from Rookie to Elite. However inactives will clog that up and can cause random patches where promotion is just unavailable. If managers leaving only drops the number of promotion spots, the league will be more open to managers staying to compete as the number of teams getting promoted from rookie should usually stay high (since good managers can regulate this).

Set the first set of promotions up as usual with a minimum number needed of say 20 drivers before promotion opens up. 

I agree with your point about the situation of Pro level.
I think it would be better having only two levels.

I think it is very important, to avoid leagues becoming stagnant, because players get kicked from inactivity.

I find it much simpler to limit the manager level for each division. Example:

To move up to the Pro debtor category at least level 12 and for the elite, at least level 16.

In addition, the IGP Deverià give bonuses for staying in the same league.

6 months = 5 tokens
1 year = 10 tokens
2 years = 20 tokens

I like the idea, but the levels you suggest are too high without removing the ballast first, which never really made sense to me anyway in leagues with pro/rel systems so I'd be fine with seeing it go. Limiting would be a good way to stop players getting promoted while massively under leveled though and I like the idea of loyalty tokens too.