I'm gonna follow up my post from before about Blender. Keep in mind you can actually have more than one install of it, so it's completely possible to maintain separate versions. I have openshell and made shortcuts to a workshop folder like this, so I can just run whichever one I need ezpz.

Blender 2.83 and on have a first-time setup that lets you make it work like 2.79. I haven't tried this so I can't give you an answer on whether or not that's good.

Make sure you download it from Blender.org. Steam does have Blender but it's pretty shit because of how blender manages user preferences.

Unity Hub is just a convenient tool for managing multiple versions of unity and projects that is recommended by VRchat itself. It isn't necessary but goddamn is it helpful. 

GIMP is probably going to be quicker and easier when you're doing texture edits, but there's nothing wrong with Photoshop. I have both because there's some things I need photoshop in specific for but I use GIMP 99% of the time. 

Substance Painter is sheer autism but if you can buy[spoiler]pirate[/spoiler] it, it works really well. You can make fantastic looking texture edits to just about anything with it and is worth checking out if you're just bored. If you go into it expecting to do something right away, you're just gonna get confused and mad.