The Mac Mini is basically a "headless" mac. Be it, a headless iMac or a headless Macbook Pro. The 2018 Mac Mini uses desktop CPU's like the iMac, now, which is a change. So, in that regards, it is closer to a headless iMac. But, the 2018 Mac Mini, in terms of CPU power can be configured to be faster than the fastest current iMac since it can be optioned with a 6-core/12-thread Coffee Lake i7. I believe the current iMac's fastest CPU option is a 4-core/8-thread Kaby Lake i7 (the 7700K)....
That can change whenever Apple's refreshes the iMacs to the new Coffee Lake 8th Gen or 9th Gen CPU's....
The Mac Mini's integrated GPU is its weak-point versus the Macbook Pro and the iMacs. I don't use Lightroom so I can't say for sure how an iGPU will affect one's work. But, my gut instincts tells me that you can do it with a 2018 Mac Mini. You're not playing Battefield 1 or BFV on it. And, you're not editing 4K or 8K on it. Just still images. Albeit, RAW and big. But still one image at a time.
I am not being sarcastic here when I say, Yeah. Totally! One image at a time is something the 2018 Mac Mini should be able to handle with ease.
Just make sure you have at least 16GB RAM for those huge RAW files. And, a fast SSD to work on, if not the internal, via external TB3 SSD or something. The latter cost more. But, I believe I/O performance (your storage medium's speed, for example) and RAM (since CPU is current) would be the more on the priority list for single, one image at a time, whether they're 30GB RAW files or whathaveyou.
The 2018 Mac Mini's integrated intel GPU can also lend a hand with openCL and Metal tasks, depending on how Lightroom is optimized. And, it should be leveraged in photo-editing work since it should have free resources for the application since it only needs to drive the display and the other cores even in an integrated GPU will be freed up to lend a hand, sorta, speak in less demanding workloads, such as photo editing.
So, my answer is a resounding, Yes. The Mac Mini 2018 is good enough for like a National Geographic Hot Shot Documentary Wild Life Photographer. I am not one, but, I'd be willing to prove it if NG hires me and gives me a 2018 Mac Mini and tickets to wild life sanctuaries.
Thank You!
And, oh, a top of the line medium format digital camera and large format 35mm digital camera... Thanks NG!
And, if, for some reason you need more oomph in your GPU, the 2018 Mac Mini has like 4 TB3 ports where you can connect an eGPU....