Code check: Installing a garage security flood light
A surface mount box can be attached to the exterior siding (what is the siding?)
In some cases I like to cut the siding an inset the box and screw it to the sheeting
OR cut through the sheeting and make the box flush mount (old work box could be used in this case).
Since you have the UF a surface mount is fine I usually use 2 screw clamps, cheaper than a CGB and 2 screw clamps are listed for the UF. a CGB may or may not be listed for use with flat cable it depends on the seal some are hard plastic and those are not on the ones I have used and some are a soft neoprene and some of those are. But I would use the cheaper 2 screw clamp.
All the unused holes are required to be plugged if you are concerned with water accumulation it is acceptable to drill a drainage hole in the bottom of the box.
The maximum hole size is 1/4”.
As far as height NEC 404.8.A  all switches and circuit breakers used as switches limit the switch to 6’7”. There are switches that can be up high but these are a specific type not a common snap switch.