How Can I Turn Off Or Disable Unnecessary Apple Programs From Running In Background?

najrellim wrote:

Is there a way, say using Terminal or Services, to shut down or disable everything that Activity Monitor shows running except for those applications directly related to audio production?

No.

If you go with the Linux alternative, make sure you have a game plan in place just in case you want to reinstall macOS again. While a 2012 Mac should have access to the online macOS installer in Internet Recovery Mode, it all depends on your local network & Internet being able to handle it. I would recommend creating a bootable macOS USB installer using the instructions in this Apple article before you erase the Mac (best to test the you can actually boot from the macOS USB installer as well):

How to create a bootable installer for macOS - Apple Support

Also, make sure your files are stored on media accessible from other devices and/or operating systems. While Linux can usually access most Apple file systems, sometimes there may be some limitations such as not being able to write data to a volume, or it may require a little more work to allow read access to an APFS volume, etc.

If you have a USB3 SSD, then you could test drive the Linux setup from an external drive first to see whether it will work for you since a USB3 SSD should provide equivalent performance to an internal SATA III SSD (make sure the drive -- enclosure, drive dock, adapter supports UASP for best USB drive performance).

Make sure all the cables disconnected to remove the LCD Panel are fully reseated. Usually there are about four of them. Usually there is one going to the power supply. If the iMac powers on, hear the startup chime or fan, or possibly even hear the optical drive mechanism move a bit, then most likely there is an issue with the LCD Panel, cables, or connectors.

There are also four diagnostic LEDs on the Logic Board that can assist in knowing at what stage the power on process fails. I don't recall if they can be seen through the bottom vents or if the display has to be loosened to see inside (don't touch the exposed power supply while powered on!) as I have only worked on a few of that generation long ago.

Oops, sorry. I was helping someone else with a similar situation involving an iMac. I'm glad you got it sorted out.

FYI, you do have to be careful with the antenna cable to make sure to tuck it out of the way to the side (push it through the gap by the antenna connector and the case) making sure the antenna is not beneath the screw hole for the antenna plate, otherwise the screw may damage the antenna cable.

I'm not familiar with Linux Studio but i have run about 50 flavors of Linux and it's not possible to disable all processes not directly related with any of them.

Apple also does not expect their users to ever upgrade the internal components so why do anything but the bare minimum to make everything fit together during manufacture?