etrecheck disc failing

I should best leave alone unless someone still thinks it's a ticking time bomb or maybe just a reason to buy a new iMac.

Well said. I've been inside many different Macs but the thought of diving into the Late 2012 and newer iMacs gives even "Mr. Happy Under-the-Bonnet" a case of full-body judders!

And I don't think you have a ticking time bomb, just the standard 21.5-inch mech hard drive slowness. 21.5-inch iMacs have shipped with underperforming mechanical hard drives since 2011. The computer has a fast 6GBps hard drive interface but Apple puts slow 5400 rpm, 3GBps mech drives in the entry-level models. They still do.

If you eventually go for a new iMac, ship the mechanical drive and the Fusion drive and go for the Apple SSD option. Not cheap but you will not regret it. I bought a new 2017 5K 27" iMac last fall and "bit the bullet" to pay more for the SSD option. No regrets.

Do not buy the additional RAM from Apple. Get the basic 8 GB. You can get the additional RAM from either MacSales.com (OWC) or Crucial.com for less than ½ of what Apple charges That is as long as you get the 27" iMac model. The 21.5" model is not user upgradeable for RAM.

Before ordering a new drive rerun Etrecheck. It can give different results depending on what's running at the time. I would definitely have Apple give it a free diagnostic check at the nearest Genius Bar before opting for a new drive. Apple has much more sophisticated testing to detect failing drives and other hardware issues.

You are NOT running out of RAM. You have 16GB of RAM. That should be more than enough.

Your hard drive IS failing. You can easily replace it with an SSD for much better performance.

Here's a complete kit for a 1TB SSD including tools and everything you need for under $250: https://eshop.macsales.com/item/OWC/K21IM12HP1TB/

Your iMac has plenty of life left in it.

That said, I'd get rid of some of the junk you have installed, starting with the VPN. It's pointless and is doing absolutely nothing to protect your privacy.

Understand that a 21.5-inch 2015 iMac was not designed to be opened. The case is glued together with special tape.

MegaMac, being an OWC partner should able to advise you on your options. Working inside a 2015 iMac is generally beyond the typical home user. Even for the kits OWC sells, they recommend professional installation.

OWC has install videos. This is for changing the hard drive in your iMac model so you can see the issues. Note the caveat above the video:

OWC Skill Level: "Advanced" 

Professional Installation Recommended Contact Customer Service

https://eshop.macsales.com/installvideos/imac_21inch_late_2013_hdd/iMac16-2/

Have you run Disk Utility > First Aid? Be sure to run it from Recovery Mode(command-r) or it won't be able to a full disk repair;

About macOS Recovery - Apple Support

::taps!::

Follow KiltedTim's advice. OWC is an excellent source for all things Mac.

OWC has UK partner:

https://www.megamac.com/collections/owc