Why is my iMac 27” running slow?

I know my late 2012 iMac 27” is like 100 in computer years, but if I can make modest investments in this machine and get it to last a few more years I will.  I’m willing to wait a few extra minutes for it to start up, open files etc, but lately it has become impossibly slow, like spinning the beach ball for new safari windows, any time I right-click, lagging behind typing etc. I want to understand why computer runs slowly when its performance numbers don’t seem to be stressed.

Some questions:

• Does the activity monitor really show valuable information? my CPU (2.9GHz quad-core i5) is almost always 90% at idle, it won’t even use all of the upgraded RAM i’ve given it (24gb) I realize there are more factors than these, but will need to be enlightened.
• My older machine is no longer compatible with the newest OS X. (I’m running 10.15.7)  Is this what will ultimately make my machine obsolete?
• The only performance metric that I can see being maxed out is the hard drive, which sometimes shows it reads/writes volume pegged.  The recent performance degradation however is not necessarily when the hard drive is extremely busy.  Would it do me any good to install a SSD?
• Would a faster processor (I realize this would mean a new computer) really do me any good if mine claims to be loafing along all the time?
• If the graphics card could be to blame, is this something that a reasonably handy person could DIY? I understand the card is replaceable on my machine.  I’ve replaced iPhone components etc, so am not a total newbie to working on electronics.

The Specs:

(27-inch, Late 2012)

2.9 GHz Quad-Core Intel Core i5

24 GB 1600 MHz DDR3

NVIDIA GeForce GTX 660M 512 MB

My 1Tb SATA disk is just over half full

As I type this (on the native Notes app) the beachball of doom spins for 30 seconds at a time.  Clicking away from notes to Safari, or another program often puts me back in the game.

I’m looking forward to the responses and to learning.  Thanks for sharing your expertise.