well for a start you have a pump which pump up to 800 gph
and a drain pipe which can only handle 660gph,you need to up grade your drain pipe to at least 1".You also have 2 1/2" hoses which are easily blocked and will cause you problems later,change these to 1" hose or try to buy just the syphon pipe from lets say an eshopps.These overflows also work very well and are quite reasonable to buy.It is imperative that an overflow syphon is built correctly if your going the DIY route,You could even make the u pipe with 1" pvc
using long radius bends to help with flow rate.
1. the drain pipe must be rated at a higher flow rate then the syphon pipe/pipes
2. The pump after head loss should be rated at a maximum of just below the gph
of the syphon pipe( if pipe is 800 gph
get pump after headloss pumps 750gph)
3. the pump after headloss minimum gph
should be no lower than 85% of the syphon flowrate(if syphon pipe is 800 gph
then pump minmum gph
4. You can put a valve into the return pipe to throttle down the flow but never put one in the drain pipework.
The flowrate going through the syphon pipe needs to be as high as possible so the flow pushes any bubbles through the the other side otherwise an air break will occur and flooding.
hope I've explained this enough for you