Deeply, deeply vanilla here.

gnome-system-monitor for network, drives, memory and CPU usage. Sensors Monitor applet in Cinnamon (which makes use of the lm-sensors console command in the background) and occasionally xsensors (likewise) for temperatures and the like.

Bottom right of my second monitor, on a Cinnamon panel, I have the aforementioned applet set to show G: xx C: yy where xx and yy are the current max temperatures of the GPU and CPU respectively. Tooltip hover on that is set to show drive temperatures, but I rarely look at those because they rarely ever seem to get above 40C.

I've also used the bash-sensors applet in the past to do similar things.

xsensors/lm-sensors needed a kernel module loaded to access extra temperature stats beyond the basics, but most of those turned out to be useless, wrong or not worth worrying about.

As for stress testing, I have a shell script that spins up several do-nothing, busywork scripts and will push the CPU to 100%, 80C and the fans to high RPM in barely any time at all. I don't feel the need to do that very often.

For the GPU, I can just remove any FPS limit from whatever relatively recent game I'm playing. Heck, even Minecraft.

I've considered installing one of the applications that adjusts CPU fan RPM curves, but I set a profile I liked in the UEFI over a year ago and that's been working fine for me. If I've ever hit a point where the CPU has had to rate-limit, I haven't really noticed the drop in performance.