It is slightly more complicated than that.
Adobe has done a good job of using your computer's GPU to speed products like Premiere Pro and Photoshop. However, it hasn't given Lightroom the same love, and with the enormous 60-megapixel-plus files coming out of cameras (and smartphones) these days, you need all the speed you can get...
www.engadget.com
There is a notion in some of the responses above that Adobe is going to add more 8 core support so going i9 will help "future proof". Well they are also highly likely going to be adding more GPU support in the future also. The tool-mix that you mainly use are cemented into the x86 core compute only era then i9 might dominate. But for tools that do tap the GPU the gap is bigger if "trade up" on GPU. And Adobe is generally "late to the game" among other photography tools in leveraging GPUs. However, some folks have a stock of "favorite" filter/tools/plug-in that aren't going to change and will remain CPU bound. So there is a bit of it depends upon what you plan to use in the future factor there.
Apple has three iMac 27" base configurations on the store page. I think the middle one presents somewhat of false dichotomy in its build to order (BTO) page because there is zero choice of GPU as though it doesn't matter. It also puts a larger gap between 3.1 i5 and 3.6 i9 . The top base config has a 3.7 i5 versus a 3.6 i9 ( the i9 has a bigger core count gap than a clock speed one. ). On the top on you can get to a Vega 48 for about the same amount of moving from i5 -> i9 . the 3.7 i5 is just plain substantively faster than what you have now. If locked into older plug-ins that can't go multicore much then just as fast as the i9.
As for the current SSD's, they just smoke your 3TB Fusion drive. ( and the current Fusion drive options). The 512GB one is probably incrementally slower than the 1TB version. The problem is Apple's $/GB pricing is a pain. I wouldn't pay lots extra for TB's of Apple SSD capcacity over a better CPU or GPU. [ And the RAM is basically a non starter too. ]
So basic priority order > 1. at least minimal SSD internal , 2 better GPU options and then if budget still left i9.
If have a limited budget, I would look for a solid enclosure that you can put a 2.5" SSD into that's USB 3.1 gen2 (or possible Thunderbolt v3 and M.2 form factor ) . The $/GB you pay for those these days is way better than what Apple is selling and are more suitable for a large photography media library/database.
If your capacity needs are constantly growing then get an enclosure and moving to bigger drives 2-4 years down the road when you need to is probably would the additional initial outlay. Bigger SSDs will be less expensive in the the future. So buy bigger closer to when needs are bigger.
Something like this could consolidate your default external drive context.
4 bay enclosure.
https://eshop.macsales.com/item/OWC/MEQCTJBT00/
Just run it JBOD ( as independent disks). You could have two backup target drives , one 2.5" SSD older stuff photography library , one 2.5" SSD newer stuff photography library. ( or just start off with just 3 or 2 and move to more over time. ). That would be faster than what you have now. ( Thunderbolt M.2 drives would fastest but also substantively more money. )
That Sandisk drive would be handy as a "on the go" drive in the field with laptop/iPad Pro. Its portable features are good. However, as a constantly hooked to a single, static desktop system's drive, there are probably better options.
