Yes, the DLC for Rainbow Moon are just small boosts in comparison to end game stuff, you don't really need it and it isn't pay to win.
As Shuklar said, it's basically just premiums you can obtain to help support SideQuest Studios and EastAsiaSoft more than you have; an extra little 'thank you' (if you will) to the developer and publisher if you really like the games and content they provide.
I personally bought all the DLC, because I felt the game is worth more than I initially paid for. Money well spent, IMO.
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