Hi Andrew, welcome to the forum.
I've just completed being a "hybrid" pond with a bottom drain and a bog filter, here is my
build thread
I have 2 x 3" bottom drains and a intake bay, mine feeds a drum filter that feeds the bog.
I'm not too sure of my total flow rate, I have two variable pumps a 8000 gph to feed the bog and 5200gph for circulation, but both are only running 50% power.
so I 7000 - 8000 gph, so flow rate is a little low but all seems ok for now, if I do need to flush, i could always turn two ball valves off to force all flow through one bottom drain.
Pipe size, I had a lot of people telling me I need to use 4" pipe for bottom drains, but I didn't think I had enough flow, so I scaled down to 3" pipe, I'm with
@GBBUDD, faster flow rates will help keep the pipes clean, 2" could work better, but could block easier?
What I don't see is a skimmer or intake bay to take surface waste from the pond, I recommend you make one, in my case I built an intake bay, it works great.