Upgrading 2017 iMac

Do you have an external drive where you store videos and use for cache files? If not an SSD of 1 or 2 TB from OWC (MacSales.com) would help some. You already have sufficient memory and may need more storage space with lots of free space for cache and swap files.

If the software you use doesn't let you select where to put cache and swap files you could move some of the files from your internal SSD to the external SSD to free up maybe another 100 GB of space on your boot drive to handle those files

And save files to the external that will have plenty of free space and won't have to search for bits and pieces of the drive to store the file to.

Note that this Mac has Thunderbolt 3 ports – so it is possible to get very-high-speed Thunderbolt 3 / NVMe SSDs on which to store the video projects that you are actively editing.

An official external SSD would be wise, an internal SSD not so much!

All 500GB storage capacities for the 2017 iMac are SSDs.

iMac (Retina 5K, 27-inch, 2017) - Technical Specifications

So, you may have a larger capacity (1-3TB) with a Fusion Drive. Disk Utility can confirm what your Mac is equipped with.

-Jack

It sounds like you might already be using an SSD? You can check in Disk Utility. To do so, in the Finder, choose Go > Utilities, then double-click the Disk Utility icon. (Disk Utility is located in the /Applications/Utilities folder.) Alternatively, click Launchpad in the Dock, type Disk Utility in the search field, then click the Disk Utility icon. 

-Jack

Hey there!

What kind of storage does your Mac currently use, and how much storage are you using out of its available capacity?

-Jack