I use my desktop at home, but i'm looking to bring a laptop to my mom's house.

I want 15"+ matte screen. I have a 500 GB SSD already. i don't want a chromebook

uses:
play movies from itunes (hdmi to a tv)
basic browsing (using my phone's tether)
diablo 2
google docs
youtube

https://www.amazon.com/Acer-Display-Graphics-Keyboard-A515-43-R19L/dp/B07RF1XD36

I came across the acer aspire 5 just from googling. Looks like it has an extra 2.5" slot i can stick my SSD into. as far as i know, i can get rid of the windows S mode crap

$313 for the AMD Ryzen 3 3200u with IPS panel, then add a bit to upgrade the RAM to 8GB. is that ok for my needs? would an extra 50$ or 100$ greatly improve what i get? are there better laptops in the price range?

that acer laptop would be fine for your "listed" uses
you wont get a cheaper laptop than a unit with a 3XXXu right now