if you go to the premium section and look at the benefits it literally says (from memory) if you pay 3.5€/month you get a small boost on interactions for your tweets. If you pay 5€/month you get a bigger boost on interactions if you pay (I don't remember how much it is) 22€/month ~ you get the biggest boost for your tweets.

That to me sounds like if you don't pay, you're going to be in the bottom of the feed for everyone else.

I just looked and it doesn't say that. It only says that you have a higher chance to show up in front of other users in the reply section. It does not say that Premium Plus gets more of a boost than Premium. It is not a perk of Basic.