Check your local Craigslist or other classifieds for cheap old GPUs. Or if you have another PC, borrow the GPU and power supply just to get it to boot.

And also are sharp enough to cut your hand if you hold it in the wrong way.

You can check the data the drive itself keeps about disk errors. This data is called SMART.

https://www.howtogeek.com/134735/check-ssd-or-hdd-health-with-smart/

Figure out if the drive is having problems or not, then look at replacing it.

Another thing that slows down old laptops is thermal throttling. Once again there are plenty of programs you can download to check component temperatures. Have you cleaned dust out of the inside of the laptop and re-done the thermal paste?

You can do your own oil changes if you’ve got the time and tools. If not that, I’d find a local mechanic that you trust and get all your work done there.

Chain quick oil places usually get things right. But they don’t have the tools or skills to fix things if they mess something up. They also tend to employ bottom of the barrel people who tend to not care about their work.

Sounds like you learned a lot. As far as the belt tensioner and not being able to budge it, usually stuff like that is a matter of leverage. You probably just need a breaker bar. Those are basically a longer version of a ratchet and are very useful for tight bolts. Usually they are pretty cheap too.