2012 21.5inch Troubles - Upgrades?

[TyroMac]

New to this forum and my first post - Treat me gently

I have a Late 2012, 21.5inch iMac with 2.7GHz Quad-core Intel i5, 16GB 1600MHz DDR3 RAM and 1.12 TB Fusion Drive. Running Catalina 10.15.6.

Until recently this setup has been running perfectly well but now I am having issues.

Firstly the system has started to load and operate very slowly with functions like page closures taking up to 10-15 seconds to complete.

Secondly, the HDD part of the Fusion Drive has become noticeably noisier of late.

I am loathed to scrap the iMac completely partly due to a replacement cost but also I like a challenge.

I am perfectly happy to get into the guts of the machine and replace components so what are the best options?

Would removing the HDD and replacing it with a 1TB SSD alone be sufficient to bring this iMac back to life? If so, would I need an adapter to connect the SSD or is it a straight swap as I've seen on YouTube videos?

Or, should I hook up an external SSD through the USB2 port and somehow (if possible) disconnect the HDD?

I am open to sensible and practical suggestions.

 

[JDiggyDawg]

Swapping out the Fusion Drive with an SSD would be a really good upgrade to go with. SSD's take the same connections as HDD's, so as long as you can get into the computer to swap out the parts, that's the route I would go.