Cj, thanks a bunch for the replies. Lots of good info to think about.
I am planning constant flood, or at least I should say the water will circulate 24/7 with no timers. I'm aware of bell siphons but I'd rather do constant flood to reduce the temperature fluctuations that come with flood/drain cycles.
Level growbeds... that is something I have pondered as well, but decided it was better not to. In my thinking, water seeks its own level. So with a small input relative to the growbed volume, the water should flow down through the bed and exit at the sump, never really rising in the growbed. That was my line of thinking. It could be dead wrong.
The diagram may not be good enough but I don't think the tank will ever empty. I'm putting the pipe at the top. Worst case if the pump dies is I lose 6" of water from the top of the tank and the growbeds start to dry out.
I think you're right that this is CHIFT/PIST. I had kinda forgotten about it but now I will follow your link and read up on it again.
Thanks for taking a look!