Action economy is super important but...those kobolds could all be dead before they even get a turn. So I dunno - it really depends on the context.
I would go with the adventurers, Beholders, and Solars. The adventurers are the most flexible, and since it seems like I get to choose, I am going to make sure that there is a Moon Druid and at least 2 wishes in the mix. Those Beholders will add a ton of control, and Solars are unbelievably mobile and heavy hitting.
Edit: Like, how do 500 kobolds take down even one Solar, let alone four? It has 150' fly, and 120' teleport that it can cast for 1 legendary action, and a searing burst that will automatically kill every kobold in a 10' radius, so every turn it can just take out a chunk of them with total impunity. Having 500 actions doesn't do much in that situation.