If it were me, I'd be leaning towards purchasing a new MacBook. Your iMac is 10 years old, and has been "left out" of MacOS improvements for several years now.
If your Photoshop/Lightroom work doesn't involve a ton of batch editing, then I'd say the MacBook Air might be the way to go. It's got plenty of power for photo editing, and (for me, anyway) the 15" size is pretty much the sweet spot...not too big, not too small. You said you back up to the cloud and your 8TB external drive, so it doesn't sound like the internal SSD size is a huge issue...you've chosen the 512 GB size, which is what I'd also recommend. If you do a lot of batch editing, then I might recommend bumping the RAM up to 24GB.
But please note carefully the specs graciously provided by Servant of Cats. The biggest limitation of the MacBook Air is that it's limited to supporting one external monitor. I mention this because we often see questions in these forums from folks wanting to know how to connect multiple external monitors to their new MacBook Air. You didn't mention what external devices (other than the external HD) you would be using, so the additional ports on the MacBook Pro might be something to look at.
For me, I was torn between the 14" MBP and the 15" MBA. I chose the MBA as the screen text is a tad easier to read on the MBA (due to screen pixel density differences).
Hope this helps.