Just a couple of thoughts. Depending on where you put the screws, they will be of little or no value. If you put them parallel to the ground and suspend the planter box on the legs using the screws, likely the screws will fail. Pocket screws are not known for their weight bearing compression strength.

Unless you buy stainless screws, they will fail quickly from staying wet. The normal screws are not made to be used in construction that causes them to be constantly wet. They will rust out.

There is a reason that mortise and tenon is used in this kind of project. The wood literally has to rot out for the structure to fail.

I have made decorative planters before, and I lined the planters made for a commercial decorative accents with aluminum with joints of the liner sealed with elastomeric caulk. The ones used at the house were lined with heavy duty plastic.

Liners keep the wood protected. Don't over water and you won't have to worry about weep holes.

Robert