Just going to speak to this point. In a previous post I mentioned that device promos are often only for the base capacity of a device. It's rare when you're allowed to apply a promo to a device with the max capacity.
Likewise, a lot of carrier deals for plans assume a base offering. The four person plan you mention? I assume that's for four lines right? I have a family plan with T-Mobile that currently has 10 lines for a four person family. My wife and I each have three lines for ourselves, two voice lines (for two phones) and one data line (for iPads). There are two lines for SyncUP Drive devices in our cars which give us mobile WiFi. And the last two lines are actually for our kids (voice lines).
Any deals I've ever found from my carrier (or other carriers) are outright assuming three or four lines max for a family. You mentioned where people spend their $$$. This is where I spend mine: on devices, on cellular plans, on my ISP, on my streaming services, etc. And that's where I am often screwed for promos because not a lot of service providers offer much more than the basics.
Ive been able to buy 256gb iPhone Pro Max from T-Mobile before and was able to take advantage of the $1000 bill credits. the 256gb Pro max was $1200 so I just had to pay the $200+taxes up front.
Also I have 8 lines on Magenta Max (but only 4 people) and had no issues with anything (6 devices on EIP). Sorry but I dont know what you mean with this part: "Any deals I've ever found from my carrier (or other carriers) are outright assuming three or four lines max for a family." What t-mobile plan do you have? Simple Choice?