I would usually not be concerned about old hardware as long as it can run a modern OS I trust. This means most laptops are fine, but phones not so much.
This is not a good suggestion because even with up to date and modern operating systems, hardware vulnerabilities can often be leveraged
Security is not guaranteed by simply using linux. It may even worsen it.
The problem lies with Jerboa, not lemmy.
ActivityPub consumes a lot of resources, so the cost for each instance owner will still be high
Fedora 38 with Wayland has given me the smoothest and most trouble-free Linux experience. I suggest using either the gnome or KDE version with Wayland.
Older machines can be at risk from hardware flaws. But if your machine can run Windows 11, which most machines from 2019 can, you can safely use and install Linux on it. I recommend not buying a new machine that lacks official Windows 11 support even if you want to run Linux.