I was using a 2015 5K for graphic work in a print environment about a year and a half ago still. Was a solid computer, it was maxed out when I bought it brand new. I did opt for the flash drive when I bought it which extended its life significantly. I still have the computer at home but really don't do much with it as I have an M1 Max and a 5K 27" screen at work.
The one thing you have suffered with needlessly is the RAM. The 27" iMacs have user upgradeable RAM. It is a little slot by the power port in the back of the computer. Super easy to add ram, takes less than 5 minutes, no screws. If it had a flash drive I would say up the RAM and keep it going, unless you can't live without being on the latest version of Mac OS.
Upgrading to an M series Mac is a big jump in performance on the CPU side alone, add that you will have a computer with a flash/SSD drive you will be amazed at how quickly boot ups are and how quickly things load. I have been spoiled with it since the 5K was originally released and can't understand how anyone has been using regular hard drives since that time.
You could get a refurbed Mac Studio:
M1 Max brings power to tackle challenges of almost any size. Whether you’re running multiple apps, sorting and editing thousands of photos, recording and mixing professional-quality music, or discovering a new exoplanet, the screaming-fast M1 Max has your back.
www.apple.com
and a Samsung Viewfinity 27" 5K screen
If you have money left over you could up the hard drive to 1TB. With that setup though externals are cheap.
Or you could go with a MacBook Pro and the Samsung screen, get portability as well as a nice big screen for working at the office/home.
Supercharged by M2 Pro or M2 Max, MacBook Pro takes its power and efficiency further than ever. It delivers exceptional performance whether it’s plugged in or not, and now has even longer battery life. Combined with a stunning Liquid Retina XDR display and all the ports you need — this is a pro...
www.apple.com
I use an M1 Max and plug into my Viewfinity screen at work. It is a great setup and I get a powerful portable computer at the same time.