Well, for starters, if you build it you can have whatever aesthetic you want. In the process, you’ll learn how all the pieces fit together and will be in a better position to perform your own repairs. You also get warranty terms of 3 or more years on the individual parts.

With an OEM Prebuilt, you’re usually limited to a one year warranty (in the US). Not all “office desktops” consist entirely of proprietary parts. Lenovo and Dell publish their service manuals, so you should be able to see if a system you are considering uses a standard ATX Power Supply (for example). Depending on your performance target, some RTX3050 cards will run entirely off of the 75W provided through the PCI slot. They’ll also have “office PCs” with graphics cards in them, meaning they’re technically fit for playing games even though they’re bot marketed as such. Ultimately, “Gamer” aesthetic is really just part of “Gamer” marketing.

You almost certainly won't find an upgradable case from Dell or whoever. Have you looked at Silverstone? I have the PS08 and I'm pretty happy with it. It's a nice cheap compact case. It sits under my desk and I hardly even see it.

My only complaints are that because it's compact, I have to make sure a GPU will fit, and I'll probably have to wiggle it around to get it into place. And I don't really have much cooling in the case so I leave the side panel off, but that's only because it's easier than proper fans and ducting. (And quieter, too.)

I somehow missed you mentioning Fractal. My point still stands though. I've got the original Define XL. it's more than a decade old and has housed either one or 3 computers of Theseus, depending. So a pretty good ROI based on its lifespan.

It's not exactly what you're looking for but Fractal makes some solid, plain black cases. Most of their models can come with or without windows. The Define line is my favorite - they're very quiet, spacious, and pretty well designed.

https://www.fractal-design.com/products/cases/define/

I went with a tiny build in a Fractal Terra. It still fit a 7900XTX and I've been super happy with the look and form factor on my desk. If you need details about the cooler and other parts, I can share a PCPartPicker link. There's a bit of an art to squeezing all your component goodies in but it should be more than enough for a very powerful, extensible build.

If you are looking for some less garish designs for cases the Fractal Design North is one great looking option, and also some of the mini-itx form factor cases look really nice too, not too far off from some of the ones you mentioned.

That said, you absolutely can add a graphics card to some of the corporate machines, but some are much harder than others. I have not done one recently but honestly I would look for someone else's build and replicate it as exactly as possible. I have had weird issues with corporate gear before around connectors, weird pinouts, very limited PSUs, and strange limitations on the PCIe port supporting some cards but not others (HP and a generic network card crashing my system randomly). If you find that someone else has done that specific box with a specific card then you should be ok, but if you have to deviate try to make sure that you match closely what they had, such as whether the graphics card has extra power or is PCIe only or if they have to use one of the corporate style graphics cards because gaming cards won't work.

Have a look at the Fractal Design Pop Silent case, it is matt black with zero LEDs or glass. I have one myself and it's a great looking case.

As for upgrading a workstation it's easily doable but it depends on the size of the case and getting a suitable 75w GPU.