OP:
What I would suggest...
First, connect the Sandisk SSD to the 2018 MBP.
Open disk utility and erase it to APFS, GUID partition format.
The best way to get all the data into the right places would be to download SuperDuper:
You DO NOT need to buy it. It will work "for free" for the job we need to do here.
Now, use SD to "clone" the contents of the 2018 MBP SSD to the Sandisk SSD. It will take a while, but when done the Sandisk will be an EXACT COPY of the original SSD.
NOW you are ready to unbox the NEW MBP and decide what approach you want to take.
If you wish to use Setup Assistant to "do everything for you", just connect the drive BEFORE YOU OPEN THE LID OF THE NEW MBP for the very first time (because it will boot to setup assistant automatically).
Now when SA asks if you wish to migrate from another drive, just point the way to the Sandisk SSD and let SA "digest" everything.
SA will present you with a checklist of stuff to migrate, and you can proceed from there. You can bring over apps, accounts, settings and data, and most everything should "work the way it worked before" (excluding apps that need to be updated).
THE OTHER WAY:
If you DO NOT want to "do it the easy way" with SA during the initial setup, you can create a new account on the new MBP without attaching the drive.
But now YOU MUST BE CAREFUL ABOUT HOW YOU MIGRATE "by hand".
And you must take steps to avoid permissions problems.
(that's enough for now)