Upgrade to a higher amount of RAM

Unless you are using extremely resource hungry software, running lots of programs at the same time, or have some software conflicts the computer shouldn't be beach balling. I'm using a 4 year old iMac with 8GB and tend to have 5-6 different programs running at the same time. The one thing I noted you didn't mention is the type of hard drive your computer has and I'm betting it isn't an SSD.

So here's my recommendation: 1) download EtreCheck and after running it, click on this icon and post the results. Make sure you are also running the typical software you'd have going. This will let us see what's going on with the system.

2) instead of going to the hassle of upgrading RAM, if you do indeed have a rotational hard drive, let's talk about an SSD instead. In a USB 3 enclosure this could cost about $150 and do more for you than a RAM upgrade.

Some Mac models do have RAM sticks permanently bonded to the motherboard at build time and are not serviceable.

The 2017 recent models have removable RAM sticks, but are very difficult to access internally for the 21.5" model and require almost a complete disassembly to access the RAM slots on the backside of the motherboard.

https://blog.macsales.com/41360-unboxing-teardown-of-2017-21-5-inch-imac-non-retina-model

https://eshop.macsales.com/installvideos/imac-21-inch-2017-memory/iMac18-1/

Professional installation is recommended, I would contact OWC directly to see if they offer this service, or can recommend a local shop who can. Not impossible, but very involved. They sell the compatible RAM you would need.

In contrast, the 27" model has an access panel on the back of the machine:

https://eshop.macsales.com/installvideos/imac-27-inch-5k-late-2015-memory/iMac17-1/

It's very easy to determine if a Mac's performance is being constrained by available memory. Please refer to Use Activity Monitor on your Mac - Apple Support. If the "Memory Pressure" graph is frequently "red" your Mac would benefit from additional memory. If not, it won't. It's that simple.

I'm stuck with the spinning beach ball ...

That's not normal. There is probably something else going on with that Mac, because 8 GB is more than adequate to run Mojave. It runs comfortably in half as much.

You have a well-configured Mac. Take it to the Genius Bar as planned and have them find out what's wrong.

Apple started this "every part is soldered to the Logic board or whatever" some time ago. The only models still available for the user to upgrade are the 27" iMacs and the desktop Mac Pros. All the laptops, Mac Mini, and 21" iMacs are only upgradeable during ordering. That also means you have to pay Apple prices for more RAM (or SSD). And, depending on the year, the 21" can also come with a slow 5400 rpm hard drive (or an SSD for a price). The last laptop a user could upgrade both RAM and hard drive as well as replace the battery was the mid 2012 Macbook Pro. I got one of those reasonably priced models and immediately increased the RAM and replaced the slow hard drive with an SSD.

Locate an AASP in your area, please click https://locate.apple.com/

to locate one in your area.