I do agree that stuff like the RPi 4 is in a weird spot where it's too weak for a lot of things but also too expensive for the light stuff. The biggest gripes I have are the SD cards which makes data intensive tasks impossible/expensive and overall makes it so you need to think about not causing to much writing. That and how hard they are to place. Large enough to be ugly and in the way but small so they're awkward to find a good spot for.

However I think the RPi zeroes are amazing for building small but intelligent sensors like picking up when a specific bluetooth device enters a room or a small microphone to create a relay point for a voice assistant. They're super easy to program since they still run basic Linux compared to other alternatives that are more efficient sure and some even cheaper but require you to access them via COM or learn much more machine close coding. Which puts up a massive hurdle for prototyping and playing around with the possibilities.

As for using old laptops that a big ehhh for me. Find yourself a used NUC instead. Much better form factor and the same power or even better. Though if they dont need to be visible then I really do prefer a small desktop, then it can have decent fans and hold hard drives. Everybody needs a NAS right? And building one yourself is easy and they make for excellent home servers too.