? I would put 4 drains on the bottom of the tank, inside an overflow. That's 2 2-inch drains per overflow box, overflow inside the tank, 2 overflows inside the tank, for a grand total of 4 2 inch drain holes in the bottom of the tank. Also inside the overflow box, I'd have a 1.5 inch return line. I'd put a gate valve on one 2 inch drain, and use the second one as an emergency drain. As for drains on the side to drain/do water changes, etc--I wouldn't. Instead, I'd do an auto water changer with a drain in the SUMP, so I don't ever have to do water changes in the first place. That's a better way.
Now, if you want to go through the back fo the tank with your drain lines, I would run an overflow horizontally across the top of the tank--I think that's what they call a "coast-to-coast" overflow. That way, you don't have overflow towers taking up valuable floor space in your tank. Also, that leaves PLENTY of room for a beanimal type overflow--so you don't have to run a Herbie.
As for the "K1", "radial flow", I don't do any of that, so I can't comment. What I do is filter socks and a bag of matrix in a sump with no chambers. Now THAT'S easy to clean. I like easy to clean. I don't know if that'd be adequate for stingrays, but it works great for big CA cichlids.