For years now, I've been an advocate of booting and running a Mac from an external USB3 SSD, rather than ripping open an iMac (or pre-2018 Mini) and swapping drives out that way. I've promoted doing so as the "fastest, easiest, cheapest, safest" way to get more speed from an older Mac.
BUT... I don't recommend that with the 2018 Mini.
Apple's new internal SSD's are "state of the art" fast.
As I see it, the best solution with the new Mini is to buy an "adequate" amount of internal storage to boot and run and maintain "basic" accounts.
(It's not worth it -- not to me, at least -- paying Apple's high prices for 1tb or 2tb of storage, unless you have the cash to toss away freely)
Keep the OS, apps, and basic accounts on the internal drive.
If you have "large libraries" of stuff (movies, photo libraries, etc.), they can be kept "externally". Your apps will still find and access them without problems.
If you need more storage, add an external USB3 drive.
It can be USB3, or perhaps something faster (USB3.1 gen. 2, or thunderbolt).
It can be a platter-based HDD, or an SSD.
Drives like the Samsung X5 are nearly as fast as the Mini's internal SSD, but... you pay a high premium for them. More than I'd want to pay (unless I could justify it as a business expense, etc.).
The Samsung t5 and Sandisk Extreme USB3.1 external drives are getting to be downright affordable. These are realistic alternatives to platter-based external storage UNLESS you need more capacity than they offer.
When it comes time to order my new Mini (the 2012 I have is still doing fine), I'll get either 256gb or perhaps 512gb, and "call that enough" internal storage.
Anything more, I'll just plug in an external drive.
