iPod Classic not detected by iTunes

I've tried following instructions from all over the place but still having no luck so far.

I don't know what that is unless you specifically state it...

Does the iPod still work properly (play songs) while disconnected?

Depending on the Enable disk use setting, the iPod may or may not appear on the Mac's Desktop when connected. Not connecting it for two years should not be a problem. However, the iTunes app has changed during the last two years... In iTunes 12, the iPod appears as a small device button on the horizontal bar that goes across the iTunes window. See screenshot below of my iTunes window (upper left corner), with iPod's device button circled in red.

When you connect it with iTunes running, you don't see it there? If you do, click it to access the iPod's settings screen in iTunes. If not...

The iPod says "Connected. Eject before disconnecting".

That's good. It means there is a data connection between the Mac and iPod. Try putting the iPod into Disk Mode

Putting iPod into Disk Mode - Apple Support

Run iTunes and connect the iPod. If iTunes picks it up, it may prompt you to do a Restore. If you're OK with erasing the iPod, do the Restore. The iPod should work and connect properly after the Restore, if there is no hardware problem on the iPod.

If you have another USB docking cable, try it to rule out faulty cable. I don't think that's the problem, because your iPod says "Connected" and "Do not disconnect" on its screen, which indicates a data connection. But it doesn't hurt to try...

As a test, shut down (power off) the iMac and disconnect all USB devices. You can leave standard USB keyboard/mouse connected, if used. Do a Reset (restart) on the iPod

Learn how to reset your iPod - Apple Support (see section iPod with a click wheel)

Start up iMac, run iTunes, and connect iPod to a direct USB port on the iMac, by itself. Try more than one USB port. This is to see if there is interference between connected USB devices. If there is no change, start up the iMac in Safe Mode

Try safe mode if your Mac doesn't finish starting up - Apple Support

In Safe Mode, third-party system extensions (like device drivers) are not loaded. Also, third-party background processes that auto-start at login do not run. Run iTunes and connect the iPod. This is to see if third-party software is interfering with iTunes recognizing the iPod. If the iPod is now recognized, you'll need to find the guilty piece of software. Post back for some tips, if this is the problem.

If the iPod is still not recognized, I would reinstall iTunes. Download it here

https://www.apple.com/itunes/

and install it over your current installation.