Can someone help me with my EtreCheck report?

The first backup of a whole lot of source data can take days, yes.

The backup is also important for data preservation purposes.

I’d let it finish.

A Tome Machine backup can also take a while when there are hardware issues with the source or destination, or when the source or destination are slow, with the integrated HDD in that iMac being middling-speed for an HDD, and with an unknown-speed external HDD.

Fusion sprints well too, but isn’t good with heavy read or write I/O activity, including backups.

Backups are incidentally also a good test of storage performance and stability.

Since the backup is already in progress I'd let it finish first. Once you have a backup you can take your time getting rid of everything you don't need. They will be removed from the backup automatically, when Time Machine eventually needs the space they occupied.

Yes, scams abound.

Personally I agree $158 for 8 TB storage is pretty reasonable. You might consider erasing the "junk" first but given that Mac's abysmal performance it may take a very long time, and be more trouble that it's worth.

Don't let anyone else use your Mac. If they must, have them log in under a "Guest User" Account you can create: Change Guest User settings on Mac - Apple Support. A Guest User Account has no ability to effect any permanent change.

I kind of agree with John Galt. I'd usually be a bit more conservative in my approach, but...

1. You have a fusion drive - which is often prone to failure, but the file system tests ran in 25.87 seconds (which is pretty good). The write speed is a bit low, but this fusion drive doesn't seem to be failing.
2. Surfshark AV is beasting your CPU for some reason. AV is generally useless but fairly harmless - not in this case. It is chewing up resources without any clear indications why. Which leads to:
3. There is a lot of old cruft on this system that is clearly from long-past installations - system mods and unsigned files (they have to predate SIP an Gatekeeper). Who knows what is in conflict on your system.
4. You have a lot - a lot - of audio plugins and tools. Guessing you are in that field - but again who knows what is conflicting amongst those.
5. The GeoBlocking/Verification tool is almost certainly doing low-level things that are causing issues - maybe it's what is spinning the AV around its axle.

Erase and reinstall might be the best bet here. Make sure you have a good backup or two first, of course. Be selective about what apps, plugins and other tools you reinstall afterwards.

"CleanMyMac" claims another victim.

In addition to it, you don't need any non-Apple "anti-virus" products on a Mac, and to compound your challenges it appears that you may have installed the execrable "MacKeeper" a couple days ago.

These things wreck perfectly good Macs. Erase it and reconfigure it from the ground up. Don't reinstall the junk.

You can probably get by with a little less than 8 TB but that Mac is using nearly all its internal 3 TB storage. Apple has traditionally recommended a backup drive of 2 to 3x the source drive's capacity, so 8 TB is about right.

Time Machine will want the entire backup drive to itself.

If your friend also installed GeoComply Player Location, then they were probably also online gambling or doing paid competitive e-sports from your computer too. So who knows what else they mucked around with.

Here you go: Back up your files with Time Machine on Mac - Apple Support

Regarding Step 1, I'd get a minimum 8 TB capacity for that Mac.