Using it as an external monitor without Targeted Display Mode is technically possible, but it would require quite a bit of time and technical skills.
Basically, the process involves opening the device up, taking the screen and LVDS cable and removing the Logic Board (which has a built in graphics driver) and replacing it with an independent graphics driver card and driver board, one that features external adapters for things like HDMI and VGA (or what you intend to use).
Here is a post of @moomike doing it with a 2006 model: How to turn a iMac Intel into an External Monitor
On a 2015 Model it will likely be about the same thing overall, but parts may be harder to source. I didn't find anything but sketchy AliExpress listings when I searched for a few minutes just now, but perhaps with the part number of your screen on your specific device and a heftier investment of time you can get better results.
That being said, all the symptoms you've describe point to an aging (and possibly failing) HDD. Given it is 8 years old, if the HDD has never been replaced before this is a sure bet. If you would like to keep this machine running instead, I am fairly confident that replacing that HDD with an SSD will restore your use of the device and be better than it was even when brand new.