Good luck I suppose as I don't recall any Indie/Niji collabs. Looking quickly at the most popular retirees, most collab with Holo/other companies though Tsuna had a collab with a Korean Niji in the last year.
I linked Roboco because she credited all the collab members, something that many don't and it drives me crazy
Aoi doesn't even have any Niji's in her tournaments -
https://e-elements.jp/special/asgs202412/#cast. Really feels like collabing with ex-niji's is highly discouraged. To be fair I think most corpo talents don't collab with ex-talents, even in EN (ex-vshojo, stars and I guess Phase). Niji's 180+ members just make it feel more glaring.
I do hope the dam breaks just like the Holo/Niji dam when Kiara and Pomu wanted to hang out.