Whats actually gained here tho if the wishes are just spent on getting another wish?
The Shadowbard lasts multiple rounds and doesn't eat your uses or stats, so that first Shadowbard spits out Music Beyond the Spheres until it drops to make additional Shadowbards, thus giving exponential Limited Wishes. Growth rate is constrained primarily by Con+Wis scores, but it's entirely trivially reaching the eighth power per round.
Edit: Wait, no, it's the other way around, the stats contribute to how long it's doubling until it picks up a slight dropoff. With 10 Wis/Con, the Shadowbards should be able to do it ten times each, then they start disappearing, so you end up with 1024-1 when they start popping, and from there the doubling affects both portions. Which... Does basically nothing to how it explodes because the very worst case scenario of being capable of it is still half a million Limited Wishes in two minutes.
Of course, since the Limited Wish is interpreted by the DM acting the part of a Dark Tapestry entity, it's a quick way to cause a drastic breech in reality, but that fluff issue is fairly standard for TO shenanigans like this.