Here are a few things, cobbled together from the input above as well as my own experience:
1. Mash thickness. I'm doing BIAB but before I used a mash tun. I typically would have 4.25 gallons of strike water for roughly 12 pounds of grain. That's just a shade under 1.5 quarts per pound, as others have noted above.
2. Mash Temp. How close to your target did you come? I typically am in the 149 to 154 range, looking generally for 152. But that partly depends on what you're trying to achieve. Generally, lower temps = more fermentable, while higher temps = greater mouthfeel, higher final gravity.
3. Water. This is going to influence your mash pH, which will also influence the results you get. There's an old bromide that "if your water tastes good you can make good beer." Terrible advice. Your water (as an example) may be too high in alkalinity, which will raise your mash pH. Getting out of range (5.2 to 5.6 pH) may influence the success of your mash as well as the final product.
I use one gallon of unsoftened tap water (it's hard, lots of minerals) but finish up the volume with RO (reverse osmosis) water, then I add some calcium chloride, epsom salts, maybe some gypsum depending on recipe, a little lactic acid to bring the pH down, and a partial campden tablet to deal with any chlorine from the tap water.
What your water additions should be depend on if you use tap water and what's in it, plus the type and amount of grain you use. There are water spreadsheets out there that can help make these decisions for you.
4. Stir! When I did the traditional mash tun, I would somewhat vigorously (not creating a froth but bringing up the bottom grist to the top) stir at 15 minutes and at 30 minutes. This made a big difference in my gravity. So--stir. Twice.
I still do that with BIAB, at 15 minutes and 30 minutes.
5. Crush coarseness. It's a sort of truism that grain crushed at the homebrew store will be more coarse than you might desire. Some argue that's so the husks aren't destroyed which will help with lautering. But it also means it's harder to get full conversion. You might ask them to crush it twice (that will help, and I do that with my own mill), but in the long run, you really want to control your own crush. If you get a mill, you'll also find that buying grain in bulk can save a lot of money, which savings ultimately can be used to pay for the mill. A Cereal Killer is a good choice, though some also are successful with a Corona mill. Stay away from Barley Crusher.
The good news is you've done a batch, your OG isn't terrible (you'll make beer, and given the recipe, I'd wager it will be good), and you learned a lot from it. Your experience now is context into which you can place the above suggestions.
Good luck!
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