How to add another monitor to a Retina Mac computer

iMac (Retina 4K, 21.5-inch, Late 2015) - Technical Specification - Apple Support

That computer has old-style Thunderbolt 2 ports, with Mini DisplayPort connectors, that can double as Mini DisplayPorts.

It can drive a single external display. If that display has a DisplayPort or Mini DisplayPort input, that would allow the use of a simple cable. Otherwise, you will need an adapter or adapter cable.

We need the model year and display size of the iMac. You can find this information by clicking the Apple symbol in the upper left of the display and then click About this Mac. Once you have that we can tell you the cable you will need, then connect and follow the steps in Connect a display to your Mac - Apple Support

You cannot update that Mac past macOS 12.* (Monterey), no matter what hardware upgrades you make to it.

The RAM is sealed inside. You could pay a repair shop a lot of money to open the Mac and replace the existing 8 GB with 16 GB (the maximum), but I’m guessing the 8 GB you have now is enough. If you do add more RAM, I would advise buying it only from Other World Computing. But I don’t see the value in adding RAM given the cost, the likely benefit, and the age of the machine.

Getting an external USB SSD and making that your startup drive might help with speed - keeping in mind that it won’t do anything to help you run current applications or current versions of macOS.

"I'm focused on finding the most cost-effective fix possible. Is there a way to reasonably boost its speed without breaking the bank?"

You have several options, buy a refurbished Mac from Apple. You can use Apple's 12 month same as cash option if you would like. If you purchase a new Mac from Apple you can also use their 12 month same as cash option. Another option is to buy a used Mac from one of Apple's trusted reseller sites such as MacSales.com, however NEVER EVER buy a used Mac from:

• Ebay
• Amazon
• Craigslist
• Nextdoor
• Local PC Shop
• Friend, Neighbor, relative

Why..these Macs are not prepared correctly and are not refurbished with quality components.

Re: “I have the following SSD; could you advise if it would be sufficient for my needs?”

AssumIng that it is large enough to comfortably hold everything currently on your internet drive, you would need to erase everything on it - changing the partition scheme ftom Master Boot Record to GUID and the file system to APFS. Monterey is going to want a stsrtup drive to use GUID and APFS.

(128 GB seems a bit small to me … you may need to buy a much bigger SSD, even though prices right now may be about to go through the roof thanks to AI companies, tariffs, etc.)

Then it would be a matter of using Carbon Copy Cloner or SuperDuper! to clone the internal drive to the external SSD.