After reading a lot of posts recommending various internal 2018 Mac Mini SSD capacities: 128GB, 256GB, 512GB, etc. I’m almost convinced that it is worth $200 to go with the 256GB because I'd get a pretty good speed bump. But I still haven’t read a compelling reason to use the internal SSD on a 2018 Mac Mini as the “system” boot drive. It seems to be accepted by default, without further discussion.
From what I’ve read, even the relatively inexpensive external Samsung T5’s work well as “system” boot drives. Not to mention the higher performance X5’s and NVMe SSDs. External SSDs will almost certainly get bigger/better as the hardwired 2018 Mac Mini SSD performance remains the same.
I keep Macs well beyond their sell-by-date. So I’m still thinking that a 256GB internal SSD is plenty big enough, because I will probably relegate it to being a unmounted but bootable emergency system clone/backup well before I stop using the Mini.