Acceptable to add knockout to PVC conduit body

Short answer: no it is not acceptable it needs to be continuous.

The pertinent article of the National Electrical Code follows:

300.12 Mechanical Continuity — Raceways and Cables Metal or nonmetallic raceways, cable armors, and cable sheaths shall be continuous between cabinets, boxes, fit- tings, or other enclosures or outlets.

You could terminate the conduit into a junction box and then take a cable that is approved to be fished into walls (NM or MC) from there. Then make joints in the j-box. However, the j-box has to be accessible after it is installed.

Good luck with your project!

Go out the bottom, otherwise the hole or any flaw in the conduit will bring water, rust and failure into your box.

Use two conduit bodies to make your 180 degree turn. You can do this pretty tight to the box surface if you really want to. Do not strap the conduit to the box, strap it to the wall.

Use THWN-2 wire in the cable. This is rated for outdoor wet locations like this location. Use stranded, you sure don't want to pig-wrestle solid #6 (least of all 4 of them in a cable!)... and stranded will make the bends easily and pull decently. If you don't want to buy 4 colors of wire, buy white - shuck it for ground and put tape on it for hot.

Why white, by the way? Because of the rules applicable to re-marking wires smaller than 4 AWG. You cannot re-mark any color of wire to be ground, it must be green, green/yellow, or bare. You cannot re-mark any colored wire to be neutral - it must be natively white or gray. You can re-mark a white or gray to be a hot, that is the only re-marking allowed (it's intended for switch loops). Admittedly, shucking a stranded wire for ground may not work - but they sell single bare solid ground wire at modest cost.