I have done 5 gallon batches with kits in biab. If you don't mind here are some things that have helped me.
1. Double crush your grains. If it comes pre crushed, just run it through a corona mill again. The finer the crush I have found gets you a bunch of points in efficiency. I want from low 60s to hi 80s with a double crush alone.
2. Squeeze the bag. There's debate whether or not this leeches unwanted tannins in your wort. It hasn't so far, and I squeeze the bag like it owes me money. Someone said that once. Don't remember who it was.
3. Make sure your mash temps are all good and stay fairly consistent. I use propane and a burner, and if my temp drops a few degrees I fire it up and try to get the temp just under the target. It will raise that last degree or two on its own.
4. Not sure if your sparging, but I will do a full boil. I boil with 7.5 gallons for 90 minutes and it gets me down to exactly 5 gallons.
5. Have fun
What size bag do you have? Wilserbrewer has bags for sale of varying sizes. If you want to do a 5 gallon batch in a 10 gallon pot, you'll probably need something bigger the a paint strainer. I bought a yard and a half of extra fine voile mesh and had SWMBO sew it with a drawstring. Thing works perfectly. Cost was about 5-7 bucks total and about an hour of time.