Belgian golden strong ale should be decent gravity (~1.085?) at 5.25 gallons.
You'll need about 300 billion yeast
Smackpack = 100 billion yeast (90 billion viable)
Spilled 1/2 = 50 billion yeast (45 billion viable)
1.5L starter on stirplate with half smackpack = ~180 billion viable yeast
1.0L step-up starter on stirplate = ~315 billion viable yeast
You're there in 2 steps
Based on small inocculation rate of initial step, I would give it some extra time to complete (2 days at least).
The second step may take 1-2 days, although with the smaller amount it could go quick.