Here is what appears to have happened in Portugal. In about a months time, the number of teams doubled, roughly filling up 4 divisions. Since then you had the bot cleanup which meant promotion of the more active/successful teams in the D.IV leagues, and teams being abandoned. Portugal is still up more than 50%, but aren't up 100%.
When a country gains 20 teams in a week, it may actually be that 60 teams are added, and 40 become bots; and it loses 10 it may mean 30 sign ups and 40 teams become bots. In Portugal the new teams are placed in the lowest numbered D.IV leagues, while the highest numbered leagues slowly lose teams.
The proposed solutions may not have worked in this case. Consolidating leagues might not have helped since many of the teams may have become bots after the start of this season. A league might have had 14 active teams at the start of the season, and 4 or 6 now.
If there had been reserve teams, last season they would have been taken over by new owners. And now you would have had to re-establish them.
One possibility would be to have a probation period before a team was actually placed in a league. Teams might have been matched up in regular games, perhaps using a Swiss pairs format. Once they won so many games (say 5) they could be placed in a regular league. There could be a requirement for some activity to generate new games, so if someone merely signed up they would never play any games, and if someone just set their lineup for a couple of games, they would go no further.
But it is absolutely necessary that people can sign up at any time and immediately begin playing. And it is possible that teams might not think that the a probation period was really playing. If you started at the beginning of the season would you really want to have a probation period and then be placed in a league after 6 or 8 games had been played. If you started at the end of the season, it might be more workable.
Other possibilities would be to put new teams in 6-team starter leagues that played a 10-game schedule over 5 weeks, with scrimmage dates. Or in 8-team starter leagues that played a 14-game schedule in 5 weeks with no scrimmages. Teams that survived could then be placed as a group into regular leagues. Perhaps the scrimmage dates could be used as makeup dates to get more league games in.