My Intel iMac started messing up from nowhere

My Intel iMac started messing up from nowhere. It's very, very slow, takes about 15 minutes to boot up and another 40 minutes to load the desktop. The Bluetooth keeps disconnecting for 5-10 minutes and then reconnecting. On top of this, my Mac crashes and restarts after working for about 20 minutes EVERY session whilst trying the following, which took about a total of 30 hours:

I've tried Command-R, but it gets stuck on the Apple logo.

I've tried Command-Option-R, but when I try to enter my wifi password and enter it, that spinny thing just keeps spinning, and nothing happens.

I've tried adding another user to log in, but it gets stuck on the user login.

I've tried going into Safe Mode, but it doesn't work and boots as normal.

I've tried resetting NVRAM and PRAM, but it did nothing.

I've tried resetting SMC, but it did nothing.

I tried going to the menu "Erase All Content and Settings", but that doesn't work for Intel Macs.

I've tried creating a bootable USB, but I thought I had to use Extended Mac Journal, but no, it's APSF, oh no, it's HSF instead 🤬. Then I realise you have to use GUID Partition Map, but you have to click on "see all devices" to change that. Then I see information telling me I need to name the Disk "MyVolume", but then I see contradictory information saying you can name it anything.

When I try to create the bootable USB using Terminal, I get "Arguments

--volume, A path to a volume that can be unmounted and erased to create the install media.

--no interaction, Erase the disk pointed to by volume without prompting for confirmation.

--download assets, Download on-demand assets that may be required for installation."

Then I try using Install Disk Creator – this worked successfully when I was incorrectly using Extended Mac Journal and not using GUID Partition Map either because online sources don't mention it or it's outdated because Apple changes everything every 2 seconds. But since I now know to use APSF and GUID PArtion Map, it comes back with an error. Even when I try the previous options, I still get the error now.

I also tried going to the Apple logo > preferences > Startup disk, and then I remembered Apple change everything every 2 seconds.

Meanwhile, Customer Support is useless and barely suggested any of the things I tried.

[Re-Titled by Moderator]