I never got a level 15 bag. When I was looking at the item list for the level 10 bag before opening it there was no other bag listed though, so I assumed it just stopped there.
Really need to get lab to 25 and upgrade your formations. Getting 5 generals out is probably the biggest thing you can do to increase your power. Also make sure you have tiger of kai as like a crimson rider troop, I've seen other people using her in replays and she usually one-shots most things. Maybe look at troops that boost her damage even more, like the SR that throws dice for a 8% team damage boost. Seems to be a popular combo.
That's the case up until around level 30 residence. At that point the collection timer caps out at 8 hours and each time you level it you are just increasing coins per hour by like 1k. It's not a very big improvement, getting to to level 30 just makes it so you can check back on the game less often.