The "yoyo" effect was mostly generated by the LP table that pushes all guilds up.Of your 30 diamond guilds 10 are going to be knocked out and 10 more added. those 20 guilds are most likely undergoing a yoyo. And another ~40 are going to be doing yoyo between gold and platinum which may still be significantly extreme differences in play environments.
By defining a defined number of guilds per league in order to balance the ascents and descents, we would reestablish a true distribution of guilds within the leagues via their capacities.
I think the Gold and Silver leagues are underexploited and 99% of truly active guilds end up in Diamond and top of the Platinum. As long as we see guilds of less than 3 players accessing Diamond, no balancing can be considered.
With my proposal, the 2 main leagues regrouping 90 guilds, the others (micro-guilds and semi-active guilds) would be mainly in Gold and Silver.
And if it were up to me, the match winnings should remain unchanged in the Diamond league (10 FP, 25 diamonds, 50 goods, 5 units, ...) but should be just half in Platinum, a quarter in Gold, the eighth in Silver and zero in Copper. In this case, competitiveness would make sense.