"at first boot of the new mac mini what would be the best option ?"
Connect external SSD BEFORE you boot for the first time.
When setup assistant asks if you wish to migrate, yes, you do.
"Point the way" for SA to the external SSD and give it time to "digest everything".
SA will present you with a checklist of things that can be migrated.
At this point you can "go through the list":
applications
accounts
settings
data
I'd just migrate it all, and "do the pruning" later on.
But you can "uncheck" things if you wish.
If you want the keychain, I suggest you migrate at least your [old] account to the new one.