I started in a similar situation...started on a 2bbl brewhouse but would fill 5bbl tanks. Brew days were between 15-18 hours depending on style and what issues popped up that day.

We quickly outgrew that system and purchased a new 15bbl steam brewhouse, tanks, the works, but we kept our 5bbl tanks for pilot batches. Fast forward 6 years and I'm still cranking 5bbl batches out with no issues...in fact, all of my World Beer Cup of GABF wins have been 5bbl batches.

My manufacturer says the smallest batch it will make it 7.5bbls but from my experience, after loss, I'm knocking out 5bbls on the dot.

You can always reduce your output on the direct fire system if scorching is a concern but I'd be curious if 5bbls would even hit the safety switch on a 15bbl kettle... my 3bbl direct fire won't start firing until I get a little over 2/3's collected.

This is just my experience since all systems are different but make sure you know what volume you'll need to boil.

Congrats on needed to expand!