Standpipe or Side overflow - definitely standpipe
If I had to plumb through the side at the desired water level to create an overflow, I would do it using a 2 or 3" pipe (depending on the ability to seal the hole you drill). The pipe would be rotatable and would go into the bed a short distance. The portion in the growbed would have the end capped and would have a slot along the length of the pipe for water to enter the pipe and overflow to the sump (this would take care of most volume issues with all but the smallest pipe). To change the water depth just rotate the pipe.
A standpipe is still way better, since you can easily and dramatically change the water levels for crops or to get rid of vermin. If you have an option go this route or do as TCL and I have with the stock tanks before you go straight out the side.
If you must plumb through the bed on the side rather than out the bottom, you can still put an elbow and have a vertical stand pipe in the bed to set the water level if the elbow isn't high enough. This is what I've been doing on all my 100 gallon stock tank grow beds since I don't want to mess with trying to plumb through the bottom trying to get around all the bracing and the concrete blocks etc. Granted as I said before, my grow beds are really deep. I generally use 1 1/2" or 1 1/4" standpipe drains on my grow beds.
This is how my stock tanks are setup also (not quite as good as straight down but better than horizontal). Mine go through the side near the bottom of the growbed, not at the water level.
You can increase the flowrate through the SLO by splitting the outflow of the SLO. One portion would go to the growbeds and the other would have a ball valve and would go directly to the sump. This would effectively regulate the flow through the growbeds as well (so you wouldn't need the ball valve off the pump and would just have one outlet). Not ideal because of solids circulating back to the fish tank but they would eventually wind up in the growbeds. With a small sump and a large solids handling pump, accumulation in the sump should not be an issue.
It is possible that the sump could be pumped dry before water comes back from the overflow of the growbeds. You have to watch evaporation and the length of the pipes when you have a small sump in relation to the tank size. A carefully placed top up valve would help with this.