This is the way I went, I got the tiny form factor versions of a Lenovo and Dell business desktops for about 100 bucks each. If you get lucky you can find real good deals on these things most will take a 2.5" drive as well as a m.2 drive, and they’ll fit upwards of 32 or 64gb of ram depending on the device.
Just be real careful with the chromebox, an lot of them can’t run Linux and are pretty low specs so I wouldn’t want try putting everything I want to run on them. The CPU in HP gen 2 one I have is just a dual core without hyper threading. I would only even look into the a Chromebox if your thinking about Plex pass and hardware transcoding.
Also should be noted anything bought used your not paying for a Windows license though most the SFF boxes come with windows license baked into the bios.
If you have Plex pass the key for 4k streaming is hardware transcoding. Quick sync with any newer Intel CPU, like 8th Gen and up, is going to hardware transcode 4k fine. Personally I bought an HP chromebox gen 2 and threw Linux on it to run Plex then have VMs and what not on separate small form factor boxes.
I’d bet you could probably run all the services you’re mentioning on something like a single USFF Optiplex but if you want to throw drives into it for storage then the towers would be the way to go.
I work in retail, but my homelab isn’t super extensive just a nas, a Plex server, and a couple proxmox boxes.
Closet I’ve ever come to being in IT was back when I was still in college and took some a networking class and some web development classes but that was many moons ago.