I think the issue are clear to see. New regiments join the leagues and tend to disband or face a loss of attendance.
We can fix that by simply only allowing the older regiments into a particular 'Stable League' 'Old Guard' league or something. The likes of your 33rds, 77y, 59th, 63e, 15YR, 78th, 51st, 25th, 45e, 6te etc (I only know English speaking sorry).
The newer regiments should enter a separate league. Which should have nothing to do with skill level but defined on their existence age.
That way the likes of your 74ths, 93e, 17e, 7thQRC, 92nd and other newer and mostly highly skilled regiments can play without fear of them disbanding and compromising the existence of the older member league.
To conclude. Two seperate leagues: no difference or intended difference in skill. Regiments are assigned into the two leagues based on direct proof on their regimental history.
At the end the winner of the two leagues face each other. Or the top 2 or 4 regiments of each league face each other in a play off.
(I hope no one gets offended. I just little faith in half of the regiments lasting 4 months. When regimental leaders disband/remake more times then I can count. Skill based means nothing as the 59th will beat anyone
).