I'm not sure how you can look at a formula which accompanying chart has Lime skyrocket from lowest value bonus to highest value bonus, and go "Damn, this is really what we should be going with". I'd like to think that that alone would highlight the issue with the formula as is.
A good portion, if not most of, whale nations can't even spawn treasures given the 15-65% range. The largest nation sits a 15035 score which caps it at 9772. While whalehood is defined by city count and not score (and with scores fluctuating as well), if you take Grumpy as a benchmark, most of their nations can't spawn a treasure. It similarly excludes Eclipse's upper cohort (C45 and up). Rose's a bit more mixed but their larger/largest nations are similarly excluded. t$'s largest nations are likewise excluded. At a glance, roughly 45 and up with fairly tall infra can't spawn it due to score inhibition. I imagine that this city count would lower as people presumably move back to 0350.
In most cases (whenever they didn't just happen to spawn in their alliance), alliances get those treasures by buying them from whichever smaller alliance spawned them. That's why there's a concentration of them along the larger alliances. This is doubly true given that the price floor for treasures tend to be fairly high (usually something like 300m), and becomes harder to justify paying for the smaller an alliance is. I recall being hard pressed to justify the cost for one in Requiem (a small upper tier, edging whale alliance), as it didn't leave much of a profit margin after the fact.
I struggle to see how your proposal is going to address concentration in the top alliances when it it doesn't affect why this concentration is happening (larger alliances/economies benefitting from having them, smaller alliances benefitting more from selling than keeping), or how this concentration is happening.