Check the health of the hard drive by running DriveDx (free trial available) and post the complete text report here using the "Additional Text" icon which looks like a piece of paper.
Another possibility is the internal hard drive SATA Cable may be bad which is very common with this particular model laptop even when using a slow hard drive.
The EtreCheck report is showing lots of memory related issues. The memory load section of the report looks "ok", but borderline so it is hard to say, but if you are using Final Cut, then you probably don't have enough memory....definitely check the memory information in Activity Monitor like @IdriSeabright mentioned...do so after the system has been booted a while and you have done your more intensive workloads. Even if the "memory pressure" is within the Green (or even Yellow) area, you also want to see how much "Compressed" memory is being used as well as the "Swap" or "Swap Out". If these two items are showing GBs, then you don't have enough memory installed for that workload.
While replacing the old hard drive with an SSD will provide a very nice performance increase (Crucial MX500 SSD is a good inexpensive option, avoid the BX500 model), that would be the only upgrade I would even suggest since at least you could later remove that SSD and use it externally with another computer. Any memory upgrade would only be usable on this particular laptop so when you retire it the memory will be useless to you. Plus, many times with this model Mac, the one memory slot nearest the Bottom Case tends to develop cracked solder joints causing intermittent system freezes rendering that memory slot useless. It doesn't always happen, but it happens enough especially if the laptop has been carried around a lot. Plus I'm not sure Crucial has any memory for these older computers now, although I believe OWC may have it. These are the only two vendors most regular contributors will recommend for a memory upgrade since they guarantee compatibility when ordering the exact part numbers recommended by using the tools on their respective sites to identify compatible memory.
While I do like this model laptop, I do agree with @IdrisSeabright that it is not worth upgrading....an SSD and SATA Cable may be worth it depending on how the laptop will be used if you can use the laptop within the bounds of the current 8GB memory. The money would be better spent on a new laptop.
