What are the best upgrades I can do to my MacBook Pro?

Crucial.com offer multiple options for maxing out RAM and SSD's. If you access the site from the laptop you wish to upgrade, it will offer to automatically run diagnostics and tell you exactly what's possible. I have used their components in several Macs and never been disappointed.

Having said that, the problem of 'slowing down' seems to happen with age and the accretion of files and junk. I quite like utilities like Clean My Mac, but many others don't, and it can be overenthusiastic in its actions.

Once every couple of years it's worth copying your user accounts off, wiping the startup disk, reinstalling the OS, reinstall only the applications you actually use, steer clear of 3rd party utilities and enhancements, import your user account and it's like the machine when it was new.

Received wisdom is that you shouldn't need to do this, but the speed bump is undeniable. A lesser, but still effective route can be to start up from the Recovery partition and reinstall the system, create a new user account and transfer your documents into it.