Better to trade in Late 2015 iMac toward 2021 iPad Pro or keep as secondary monitor/storage?

I just got a $440 trade-in value for a late 2015 27" 5K Retina iMac with 1TB Fusion Drive I inherited from my sister when she died a few years ago. (Not sure of RAM; she may have upped it because she had a graphic design business.) Unless I use this as an across-the-room second monitor/TV setup (with Apple TV+ through Spectrum, if that's even possible) plus backup storage, I will never use it as a computer due to space issues and the fact that I work (on fairly simple book production/cover design using Serif's Affinity products) in my recliner with a lapboard.

I'm planning to buy a 2021 iPad Pro 12.9 soon and wonder if I'd be further ahead trading in the iMac and using the gift card to get more bang built into my iPad Pro...or if there's something I am not seeing here that would make it more advantageous to keep the iMac. My actual TV (hooked to Apple TV box) could use replacement, if the iMac could be used that way and integrated into my workflow...iPad TO iMac, not the other way around.

What would you tech-savvy gurus recommend? I'm not a tech noob, but I'm not capable of getting too deep under the hood either. Both the monitor resolution and the storage capacity of the iMac seem to be advantages I'd make use of, but not if I can't make it work with my new iPad Pro.

Thank you so very much in advance for allowing me to pick your brains...