Should I spend $30 on DDR3 RAM for my 2015 system?

Yes, that’s a no-brainer. A full platform upgrade costs $200–300, while $30 for 16GB of DDR3 is the cheapest performance boost you can get. It’ll make multitasking smoother and keep the machine usable for another year or two without locking you into anything. If the rest of the system still works, drop the $30 and call it done.