Welcome to MacRumors...
Though i know just little about fabric design and not much about the Mac mini (2018), I wanted to drop in some thoughts.
To produce surface pattern designs with good quality, almost any Mac should be valid, I guess it´s more on the designer to produce good quality than on the Mac. So yes, the new Mac mini should do the job, too.
As that Mac mini doesn't have a dedicated GPU, but an Intel UHD Graphics 630, it would make sense, that someone actually working with it and Adobe Photoshop and Illustrator, could share experiences about the capability of the Intel graphics card.
Probably it´s a blast from the past, but certain functions you might need were not available in Adobe apps on older Mac models without dedicated GPU from NVDIA or AMD. These were 3D, Oil Paint, Render – Flame, Picture Frame, and Tree, Scrubby Zoom, Birds Eye View, Flick Panning, Smooth Brush Resizing.
https://helpx.adobe.com/photoshop/kb/photoshop-cc-gpu-card-faq.html
The SSD is soldered and won't be upgradeable - just buy the Mac mini with an SSD of the maximum size you can afford. The some goes for the processor - take the best you can afford.
Fortunately, the RAM is upgradeable. Even though it´s a bit tricky, you could upgrade the RAM at any time by yourself, if really needed. The base 8 GB should be valid for running Photoshop, Illustrator and some other apps simultaneously, as the memory management got some steps ahead during the past years.
Because of the HDMI standards still in use at this time, I recommend the use of Thunderbolt instead of HDMI to connect the display - provided that your Display has got a (Mini) Display Port. Depending on your display, you might get better color depth.
Hope that helps a bit.
