The 2018 Mac Mini's integrated intel GPU can also lend a hand with openCL and Metal tasks, depending on how Lightroom is optimized. 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. And, a fast SSD to work on, if not the internal, via external TB3 SSD or something. Just make sure you have at least 16GB RAM for those huge RAW files. Totally! One image at a time is something the 2018 Mac Mini should be able to handle with ease. I am not being sarcastic here when I say, Yeah. You're not playing Battefield 1 or BFV on it. But, my gut instincts tells me that you can do it with a 2018 Mac Mini. I don't use Lightroom so I can't say for sure how an iGPU will affect one's work.

The Mac Mini's integrated GPU is its weak-point versus the Macbook Pro and the iMacs. That can change whenever Apple's refreshes the iMacs to the new Coffee Lake 8th Gen or 9th Gen CPU's. I believe the current iMac's fastest CPU option is a 4-core/8-thread Kaby Lake i7 (the 7700K). 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. So, in that regards, it is closer to a headless iMac. The 2018 Mac Mini uses desktop CPU's like the iMac, now, which is a change. Be it, a headless iMac or a headless Macbook Pro.

The Mac Mini is basically a 'headless' mac.

Mac mini 2018 photo editing