Should I buy Disco Elysium in 65% sale or hope it will end up in the plus eventually?

I just bought it in the sale, so no doubt it'll be in the PS Plus collection any day now. ✌️

Just buy it. Corporate malfeasance aside, the game is worth paying full price.

I can't speak for the cost relative to your financial means, but it's a really excellent game. The writing is second to none. The setting is incredible, and its only fault in that respect is that you don't get you learn more about the wider world. The game plays well, and there's very little frustrating about it.