"When I get my new mini in the mail, I plan on using the 128gb internal drive for system/apps and I have an external 1tb ssd for the user folder."
NO.
Don't do it this way.
Your home folder should be on the boot drive, with the OS and apps. Keeping it elsewhere is an "invitation for trouble".
But... your "large libraries" (such as the Photos library and the iTunes music folder) should be on the external drive.
This isn't difficult to do.
Do you have an external hard drive around right now?
How I'd do it:
a. Take new Mini out of box, set it up, but don't turn it on yet.
b. Use either CarbonCopyCloner or SuperDuper (both are FREE to download and use for 30 days, doing it this way costs you nothing) to clone the contents of your OLD (1tb fusion) drive to the external drive. You now have an exact, finder-mountable copy of your old Mac at the last moment you used it.
c. Connect the external cloned backup to the NEW Mini
d. Press the power on button for the first time.
e. Begin setup. At the appropriate moment, setup assistant will ask if you with to migrate from another computer or drive. YES, you want to do this.
f. "Aim" setup assistant at the external drive -- it will take a minute or two to "digest everything" on it and present you with a list of options.
g. Now one must BE CAREFUL about the choices. I would choose to migrate apps and settings. I believe SA offers you "checkmarks" about what to bring over from the users' folders. I would choose to "leave most stuff behind" essentially UNCHECKING almost everything IN the user folder (but still bring over the account folder itself). One thing I would bring over is mail. But I'd definitely leave behind movies, music and pics (for the moment).
h. I believe you also have the option to bring over "data" -- whether you do this or not depends on HOW MUCH data you have and whether it will fit or not. It's up to you.
What this will do is to migrate over a "bare bones" user folder.
Now, you can decide what to do with "the rest" of the contents of your old user folder.
I would set it up on an external drive, and use apps like Photos and iTunes to "reference" the libraries thereon. Just hold down the option key when you launch Photos or iTunes, and you'll see how easy this is.
One thing you'll need to do on the external drive (to prevent permissions problems):
- Click on the drive icon one time in the finder, then type command-i (eye) to bring up the get info box
- At bottom of get info, click the lock, enter your password, and then put a checkmark into "ignore ownership on this volume" (in sharing and permissions).
Personal experience:
I've maintained separate boot and data volumes for the last 25 years on my Macs.
Doing so is far FAR easier than many folks realize.
Things actually work out BETTER that way.