While your battery is getting close to its life limit (1,000 cycles), it doesn’t look like its your current issue [https://support.apple.com/en-us/HT201585|Determine battery cycle count for Mac notebooks]
I would focus in on cleaning the dust buildup internally. Making sure the fan and the heat sink fin area are clean of dust and debris that builds up over time. Use a soft paint brush and a can of can’ed air to blow away what you’ve loosened up.
I would install a good thermal monitoring app like [https://www.tunabellysoftware.com/tgpro/|TG Pro] its still one of the best out there and is reasonably priced for what it offers. It will allow you to pump up the fans RPM to help keep your system cool when you’re stressing it.
You may need to replace the heat sink as I’ve often seen the heat transfer tube spring a leak of its coolant, so the heat is not getting pulled away. Using TG Pro should help in identifying if this is the case.
The last thing is reducing your application load. Try to limit what you are running concurrently as well as =helper apps which are auto loaded. Your system is limited to 8 GB of physical RAM, so when you need more RAM space it needs to reach into your drive to leverage it as virtual RAM. As your system is also limited with its storage (128, 256, or 512 GB) you need to make sure your drive has free space for the VRAM can sit, otherwise your system has to work even harder (think heat) to work in smaller chunks of data which also wears your drive! Unlike spinning iron drives (HDD’s) Solid State Drives (SSD’s) have a limited write cycles.
[quote]Think of it this way… If you remember cassette tape players a song recorded once and played often will over time become distorted as the tape will stretch or can even snap! As that point the tape is trashed. Even if you don’t play it that often but use it to record your notes the quality of the recording degrades after many repeated recordings it has received before you know it you’re hearing the older recordings just under your newest.[/quote]
So you may need a new drive and if you really need what you have on it you should aim to have 1/3 of the smaller drive for free space, and the largest aim for better than 1/4 of free space. I do recommend sticking with the proper SSD drives for your system Apple, OWC or Transcend which are fully compatible [guide|135947].