Upgrade to 16 GB RAM and 1TB hybrid SD
I'm a developer, and my computer was having trouble handling running a virtual box while running several browser tabs, listening to music and having our company chat app open. It never crashed, but the fan was constantly going and I noticed the memory pressure was often very high.
I upgraded my RAM from 8GB to 16GB and from a 500GB spinning disk HD to a 1TB hybrid drive. The repair cost me ~$290 from iFixit (including parts and repair kit) and only 45 minutes of my time, as the repair can be done easily in tandem. In order to get a new computer for work it would have cost me north of $2,300 to get a Macbook I would consider upgraded enough for the job my work demands. For ~10% of the price, I'm thrilled to say my Macbook can handle the job like a champ now. Although I do notice the battery life is a bit shorter, so be conscious that if you're asking your existing machine to do more work in the same amount of time, it will drain the battery more.
For me the two sticking points (~3 minutes each) were getting the bottom RAM chip to seat correctly and getting the HD to connect correctly in the provided HD USB connector.
For the bottom RAM chip, it helped to take time making sure the slot was angled up so I could slip the chip in. At first I think I was trying to insert it in a way that would not allow me to push it in far enough to press down flush to the battery.
For the HD USB connector, it unfortunately is an unfriendly user design, as ensuring a solid connection between the HD and USB case is shielded by a horizontal inch or two of rubber. My only advice would be to go slowly and make sure that it is fully inserted. Mine was only partially connected initially and Disk Utility would throw an error when trying to partition the disk. This lead to a google search, and eventually trying again to seat the HD correctly and feeling it slide in. Once the connection was solid, I reconnected the USB cable and had no issues with the rest of the process!
I also highly recommend super duper, but be prepared for the process to take a while. I had about 400GB of data to copy from my old HD to the new one, and the process took ~7 hours (it is completely passive, I just let it run over night). This allowed me to boot off the USB HD and ensure it had a working copy of my OS and files before having to install it, which was immensely reassuring.