Why use any of those options and not just a number; "three days from now" etc.? Because the flow of language can feel better or more natural using some words over others. You can also notice that the way people interpret it can change based on where in the week it is. On a Monday, "this" and "next" can have the same meaning but not on Friday.

My argument is simply that "this" and "next" should be synonymous under this context since they mean effectively the same thing by that usage. It also would mean that it won't change depending on the point in time of reference.

Well I, for one, would never personally use anything redundant in speech. Otherwise my own, personal intent, may be muddled. Language has never seen pointless redundancy as it would make no sense for there to be redundancy.

Sorry but I had to tease you there. Redundancy exists plenty in common communication, and, to avoid misinterpretation, I do typically refer days in numerical values. However the mothod of using "next" seems to have a varying definition based on who you speak with and if "next" is a pointless word to use in this context, so would "this" in "this weekend". Why use many words when few do trick? People are going to add filler words for flow of speech. Its okay for it to be useless or filler, just as people use " uhh", "but um", and "like". My argument is just that even if it is redundant, the meaning shouldn't change to the way people are effectively using it as a contraction for "next week's"

Edit: I also want to be clear, this is me dying on this hill lmao, just as the OP asked haha

My Indian coworkers do something similar, referring to this week as "today week"

That sounds a little bit like a language barrier type deal but that does make more sense to me too, since you are prefacing the Saturday with a unit of time to measure by "week"

Why does it change based on the time of week lmao I'm not upset it just does not compute that the meaning of the word changes simply because the day of the week is different