Are you able to a split boil? I used to do 5 gallon BIAB all grain batches using a 5 and 3 gallon pot on my electric stovetop.
I'd mash and batch sparge in the 5 gallon pot using the 3 gallon pot to heat sparge water. All runnings would be collected into my fermenter. After I got the required pre-boil volume of wort in the fermenter I gave the wort a stir and drained it into the two pots (both would be close to full). All kettle additions (hops, irish moss etc.) would be split proportionaly between the two pots during the boil. After the boil the wort in both pots would be chilled and then recombined back in to the (by now clean and sanitised) fermeter and the yeast pitched.
This did extend my brew night a little over making 3.5 gallon batches doing a single boil but not as much as doing 2 batches back to back. For a little while the extra 1.5 gallons seemed to be worth the extra effort.
Eventually I went back to doing the smaller single boil batches just to make my brew night faster and less complicated. If my stock o' beer starts getting stretched thin I'll just do a five or six gallon partial mash brew to beef it up.