I plan to install electrical outlet in the following location:

It would be used mostly for:

1. USB camera that is always plugged in.
2. pressure washer.
3. car charging.

I plan to splice the wires from the electrical box behind the lamp:

I watched few youtube videos on how similar projects are done and If I understand correctly, I have to:

0. Turn off electricity at breaker box.
1. Buy old-work electrical box (i.e. one that has those wings and can be attached to the wall opposed to stud). PVC material should be preferred as then I don't have to use metal-sheathed cable.
2. I need to drill a hole in the wall where the new electrical box will be installed and then use a sawzall (or something similar) to cut out rectangular shape that matches the electrical box.
3. Punch a hole in the bottom of electrical box behind the lamp. Punch a hole in the top of electrical box that I just bought. Guide the cable behind the wall to both boxes.
4. Buy a 15 Amp GFCI protected outlet because GFCI is mandatory for outdoor outlets. Connect all 3 wires to it and screw in to electrical box. Splice all 3 wires on the other end to the lamp wiring.
5. Buy a Plastic Weatherproof in use cover and attach it to electrical box. In-use is mandatory as I would leave camera plugged in at all times.

Am I missing something? Do I need wire clamps too that would lock wire in the electrical box? Do I need to apply silicone caulk somewhere? Is it ok to install outlet that high above the ground? Are parts that I plan to order compatible with each other? Do I need to check wiring going from light switch to the lamp if it can actually handle 15 Amps?

The fuse in electrical panel is rated for 15Amps. At the moment I don't know how likely it is that some other device behind the 15A fuse would ever cause consumption to go over 15Amps.