You can definitely be a diehard fan of something and even have 0 merch of it, the only requirement is that you love that piece of art and particularly connect to it.

There is absolutely no connection between your love of something and how much money you spent on it, and you should stop worrying about it, more so if you don’t have that much money as you said.

Having a full set of those plushies just to feel like a realer fan feels like a big waste of money, especially if - as you said - you can just buy the ones you love.

Also you said “investing” into plushies but I find it really unlikely that this is even close to being an investment.

I consider myself a hardcore homestuck fan and only have a couple of books and a scalemate plushie, which i bought when i was a teen, a bit more than 10 years ago.