Casinos can deny KYC documents for any reason or for no reason. If they want an excuse to not payout a big win, KYC the client and reject any documents they provide for whatever arbitrary reason. Introduce a third party (who picks the 3rd party? It won't be the player, it'll be the casino) and you have the same issue, except your trust is outsourced to the third party.
I decline to play on casinos that abuse KYC. Even if they're inclined to use a third party intermediary, that doesn't solve the problem.
This is also the reason on why I only submit my KYC documents to established casinos that are already well-known and reputable in the market.
The risk is just too high for you to submit your KYC documents in online gambling casinos that are newly created in the market. Sure, they may offer bonuses that are higher and better compared to your conventional gambling websites. But, the risk of compromising your identity and arbitrary changing their TOS to their advantage is just too high for me to risk my resources.
In conclusion, if you want to submit your KYC documents, it is highly advisable that you choose a casino that is already proven its identity and worth among the players. A good example of such casino are your casinos that are heavily advertised in this forum (e.g. Duelbits, Roobet, etc.) since they have the budget to allocate advertisements, which constitutes good faith on their part.