The crank keeps coming loose and the pedals will unscrew

The thread direction of the pedals may be an issue here, are the pedals installed on the correct side of the bike?

Can you describe the looseness in the crank? Is there play in the whole mechanism (you can push the cranks up, down, left, or right)? Do the cranks fall completely off of the stem in the bottom bracket (the crank falls off entirely)?

If the pedals were installed on opposite sides, the threads may be damaged. You may be able to screw the pedals into the original threads without issue. You may need to repair the threads with a tap, (a bike shop job unless you happen to be a machinist) or you may need to replace the crank arms. (and bottom bracket if both crank arms are the same piece of metal)

If the pedals were installed on the correct side, make sure you apply enough torque to seat them properly. I have a poorly built multitool that can apply about 10-12 Nm before it feels like it'll break. 5-6 Nm is snug pressure and anything above 20Nm will require lots of force on a long tool. If the pedals still fall off with proper torque, consider using red loctite.

If the looseness in the cranks is from the bottom bracket, you may need to adjust it. See your product manual for more details on bottom bracket adjustment on your model.

If the crank arm comes off, make sure you use as much torque as you can on the crank bolt, and again use some loctite if all else fails.

In the worst case scenario, you can always replace the bottom bracket, crankset, and pedals. Consult the product manual for details on the bottom bracket. The crankset provides an opportunity to change your gearing, if you feel that the low range isn't low enough, or that the high range isn't high enough. A smaller chainring will make it easier to start and climb, while a larger chainring will let you go faster with higher force at the pedal. You can also get smaller cranks if you feel that the standard crank length is too long, and your knee gets too high on the upstroke. The pedals may need to be replaced if the threads are damaged, otherwise, they'll probably work fine.