So to make sure I'm understanding this proposal.... If a spot opens mid-season in league 1, someone from league 2 takes the spot. That counts as one of the 7 promotions / relegations, leaving 6 more for the end of the season. If a total of x players leave during the season, then there are 7-x promotions / relegations.
I like this proposal well enough, so long as x doesn't get too large. But with too many surrenders, there would be few if any promotions / relegations. I'm okay with this from the promotions side, because the top players from league 2 would have moved up to replace resignations. But from the relegations side, it seems unfair for last place in league 1 to get to stay there.
This scenarios would be even more likely in lower leagues, due to a cascading effect. If 4 players resign in league 1, 4 players from league 2 would move up. Meanwhile, if 4 other players from league 2 resign, you've now opened up 8 spots in league 2, all of which would be filled from league 3. In this scenario, which is not at all unlikely, the end result is that last place in league 2 survives.
Given where I am in the standings of league 2, I could live with this.

But I don't think it's the right answer.