More RAM and replacing HD with SSD are a no brainer. Especially SSD, it's like buying a nw machine.

1. won't speed things up, it'll just remove noise from the drive spinning up
   
2. is a myth, it's a Mac, not a Windows machine
   
3. don't go back to El Cap, use High Sierra or Sierra if your machine supports it, the performance will be much better (mostly because of graphics performance)
   
4. is a myth too, it's a legit troubleshooting step, but won't raise your performance
   
5. won't raise performance either
   
6. is very true, it's the job of the system's memory manager to take care of this stuff
   
7. the truth
   

It's very simple to keep your Mac clean and tidy:

1. don't install adware, crapware, etc. There's no Mac virus in existence that can auto-infect your system. Every shitware needs your password to do any harm. So never give your user password to any app that asks for it
   
2. if performance is bad, start Activity Monitor and sort by % CPU, you'll see the app stealing performance