Just to add a different pov - we have several Macs in our house and they’re all old - the newest being an early 2012 MacBook Pro. I’ve replaced all of the hard discs with SSDs, which has transformed them from being barely useable into workable computers again. Adding RAM also helps. Replacing the disc and RAM in the 2008-2012 unibody MBPs is easy - as StuAff mentioned, the iFixit site has excellent pictorial guides.
All of my Macs are currently running Catalina, which is still getting security updates from Apple (until the next OS upgrade at least). I have three 2009 Macs that don’t officially support Catalina but using the Dosdude1 Catalina patcher, it was pretty straightforward to install and all three machines have been rock solid since the upgrade. There are also patchers to allow unsupported Macs to upgrade to Big Sur and Monterey but I don’t have any have experience of those. At first look, they seem quite a bit more techy though.
I know ultimately I’ll need to replace them all but I hate to consign something to landfill while it’s still usable.