8gb RAM is ample to run latest OS, you only need mire for intensive photo / video editing. IME buying one 8gb chip is cheaper than 2×4 and that way you’ll end up with 10 (also I think 2+4 = 6 isn’t worth doing). An SSD will make a massive difference too. I prefer Samsung Evo SSDs (have done 5 now). 250gb is pretty cost effective.
Definitely worth doing bs a new machine which are £££ these days
If younsearch back I have posted a few times how I do the upgrades. If you can’t find email me.
I suggest you make a time machine backup, usb key start up disk and buy a cheap usb caddy to reuse / access your current hard drive
FWIW I put more ram amd an ssd into a friends 2008 MBP, that’s the oldest I’ve done. My Mini is 2009 and others I have done are 2012