Not sure if I understood correctly: "20 years" and "HDD" in the same sentence? OMG
I changed my hard drives every 2-3 years, when they're still ok, as in Switzerland the manufacturers must guarantee these items for 2years. I don't trust any brand nowadays.
For my external pocket-size drive: I trust them up to 4, 5 years, rarely more, even though they are disconnected most of the time.

For what concerns your case: 3 Sata drives plus 1 RTX3090 = quite a bit of heat.
The problem is: often, cases are thought for standard, office-like computing. If I were you, I'd invest in a better case, BeQuiet for example. I think that you really need something that pulls air inside + something that pushes it outside.
And check that your case has enough place around it.
Having built 6 PC's overtime, I know the time it takes :smile: 

Concerning the render settings: I'm doing test renders using this:

I try the OptiX mode, than I switch back to the standard GeForce mode if some props are causing crashes (it truly depends on  the materials)
Then, Poser's "Ultra NonBranch (GPU)" is a good start.
I don't know for Blender, but with Poser, don't check the "Branched Path Tracing" option using a GPU

𝒫𝒽𝓎𝓁

---

(っ◔◡◔)っ

👿 Win11 on i9-13900K@5GHz, 64GB, RoG Strix B760F Gamng, Asus Tuf Gaming RTX 4070 OC Edition, 1 TB SSD, 6+4+8TB HD
👿 Mac Mini M2, Sequoia 15.2, 16GB, 500GB SSD
👿 Nas 10TB
👿 Poser 13 and soon 14 ❤️