Well 2x 3S 4800mah LiPos in parallel will make a 3S 9600mah battery not a 6S 4.8Ah. Batteries in series add voltage and in parallel they add capacity. If you meant the two 3S battery’s in series then yeah that would work and it’s pretty much what I started out with, 6S 5Ah, though your only going to get about 4-5 miles of range which is pretty crap. If you really want your board to be practical and useful your gonna need a larger battery and a VESC though again your budget doesn’t accommodate for that. Though my words of advice are to just start out with a good board instead of upgrading overtime, you’re only going to end up spending more money in the long run. I’ve prob spent somewhere near $1.5K on my board because of how cheaply and crappily I tried making it originally that I had to scrap a bunch of stuff and re build it. If I had just done it properly the first time I’d probably only be in about $800 and have an even better board than I do now.
I used two 3s 8000 mah zippys for my first 6s board, wich got me about 10-12 km of range. If you want to upgrade later, you could throw another pack of two of these in series to get 12s (or only one to get 9s) and a vesc.