I made this exact mistake when the Switch OLED came out. I got caught up in hype, I bought the new one, and slowly it dawned on me that I just spent 300 extra dollars for exactly the same machine I already had, and a screen that, as good as it was, was NOT worth that 300 dollars.
If you don’t own a Steam deck, then go nuts! Do what you want.
If you already own a Steam Deck, you already own everything you actually wanted a Steam deck for. An OLED screen is not going to make the games you already play somehow magically better. You’re fine. Chill out
If I bought one, I would do it to be able to give my wife my old deck.