Probably the only SSD that is going to be "as fast as" the internal SSD on the Mini is a Samsung X5 thunderbolt3 drive.
BUT... they are expensive.
Unless you really want to spend the money, I'd pass.
You'll do best by running the internal SSD as your boot drive, but keeping "large libraries" of stuff (such as movies, music and photos) on an EXTERNAL USB3 drive.
You don't want this to be "a boot drive" (even though it might have a copy of the OS on it) -- rather, it should become your "primary external storage" drive.
(Aside: of course it COULD be "a boot drive" -- but it's just not going to be as fast as the internal SSD)
It could be either:
- USB3 SSD
or
- USB3.1 gen2 SSD
A USB3 gen1 drive will give you read speeds around 430mbps.
A USB3.1 gen2 drive will give you read speeds around 950mbps.
Twice as fast, yes.
But...
something to be aware of... the gen2 drives seem to run considerably hotter than do the "regular" USB3 drives (which seldom even get above being "warm").
What I'd suggest:
Get a "bare" 2.5" SATA SSD. 1tb should run around $100 if you shop carefully.
Get this USB3 enclosure:
The MiniPro USB-C offers the latest USB-C connectivity while providing the fastest data transfer rate available. The design features an aluminum body engineered to provide maximum protection and effective heat dissipation. The MiniPro USB-C is universally compatible with all USB & Thunderbolt 3. ...
www.amazon.com
(I have the USB3.0 version, it's built like a tank)
This will give you a solid USB3 external drive for storing data and also serving as an external boot drive when needed.
One other thing:
Remember that you will have TWO "primary" drives:
- your "primary internal" boot SSD
and
- your "primary external" storage SSD
So... you now have to BACK UP BOTH drives.
As always, I recommend only CarbonCopyCloner or SuperDuper for this job.
You could PARTITION the existing HDD you have:
1st partition - 256gb (this becomes a cloned backup of your internal SSD)
2nd partition - whatever is left (this becomes a backup of your external SSD).
or perhaps even a 3rd partition for just "scratch storage).