Going from 5 gallon to 10 gallon batches is pretty straightforward.
Going to 15-20 gallon batches is more complicated. Especially 20 gallon batches. You're looking at 24-26 gallons pre-boil. That would require a 30 gallon brew kettle, minimum. For the mash tun, forget about a 60 quart square cooler, you're looking at another 20 or 30 gallon kettle. Then for the fermenter, unless you want to split your batch across 4 carboys, you're looking at a 27 gal conical.
IMO, you'd be better off sticking with half barrel (15.5 gal) batches. With 20 gallon batches, your cost and complexity will go way up.