The first thing you should consider doing with you old device is selling or giving them away. This helps lowering the need for manufacturing more hardware, it prevents the hardware becoming e-waste in a drawer, and it put pressure on the market to lower it's prices.
Sure, you can reuse as a NAS, but someone probably needs it more.
The electronics went so cheap recently, so selling it to strangers is rarely worth the effort. Then there's a question, what OS are you going to put on an old PC. And then even if they are, say, only using browser, and would be okay with linux, modern browsers need 8GB of memory at least.
I know I am in the minority and my uses/needs/requirements are not average, but I am perfectly fine with running Xubuntu on the following hardware: 1) 4GB 2011 Thinkpad with HDD (yeah really) and 2) 4GB 2009 Phenom desktop (was Win10 until a month ago).
By fine I mean running all these at the same time: firefox with several tabs, development tools, Blender and GIMP. All snappy and fast. Even the HDD in the laptop is only an annoyance during/after a cold boot. Then it makes no difference. I daily drive both for the past 8-15 years. The laptop sits at ~10-15W idle and the i5 in it is a workhorse if needed.
Of course there are uses for better hardware, I am not dismissing upgrades. But the whole modern hw/sw situation is a giant shipwreck and a huge waste of resources/energy. I've tried very expensive new laptops for work (look up "embodied energy"), and Windows 11 right-click takes half a second to respond and Unity3D can take several minutes to boot up. It's really sad.
edit: To be honest I have to add a counter-example: streaming >=1080p60 video from YT is kind of a no-no, but that's related to the first sentence of my post.
