Hey all. Wanted to post an update on my testing and progress so far.
I'll start by saying I have a much greater appreciation now for the suggestions to go with an over-the-tank type filter LOL! Getting this same-level sump working has been a challenge to say the least. I kind of got into one of those hard headed "I've come this far and I'm going to figure this out" modes. Time will tell if that is the right choice. In any case I've been enjoying tinkering with everything and have been learning a lot.
Phase 1, 10g sump - 700gph fixed speed pump - discharge into sump not submerged (head pressure flow only, no siphoning). This was mostly just proof of concept and leak testing. I knew the 10gal was way too small but it's all I had on hand. As expected the sump emptied super quickly, and would only get a trickle back into the sump. I figured it would equalize a little faster, but I guess it makes sense. The level in the display tank was only rising maybe 1/2" so there just wasn't enough head pressure to push the water. Confirmed I needed a bigger sump. No leaks from bulkhead or plumbing so that was good.
Phase 2, 30g sump - 700gph fixed speed pump - discharge into sump not submerged (head pressure flow only, no siphoning). Went to LFS looking for a 20-40g aquarium to use, walked out with a rather expensive 30g ready to go 3-chamber sump. I was planning on DIYing the sump myself, but it was exactly what I needed and saved me the time. It's getting cooler at night so need to get the turtle inside sooner than later. So this setup was same as the first just bigger sump. As I hoped it was able to build better head pressure and get a better flow coming back into the sump. But with the return pump going full output it would still empty out the sump. Started playing with the ball valve to throttle back return pump flow. Was able to get the sump to a steady state, but only with the ball valve like 20% open. The pump didn't seem too happy about it, and it wouldn't be turning over enough water (I'm guessing it was only 100-200gph at steady state).
Phase 3, 30g sump - 800gph adjustable DC pump - discharge into sump submerged (siphon flow). Decided to ditch the fixed output pump for an adjustable. Wow what a difference, so much quieter and don't have to tax it by throttling the ball valve. It also will shut itself off if it runs dry, so don't have to worry about coming up with a solution for that. Should have gone with that from the start. Also realized that to get enough flow going to turn over the water sufficiently (going for 500-600gph) I can't just have head pressure flow. I'll need to submerge the discharge pipe into the sump and get a siphon going. This is what Oughtsix was talking about, changing the level at which the display tank and sump levels would try to equalize, effectively increasing head pressure. I extended the discharge piping so it was submerged just below the first chamber baffle and drilled a hole in the short horizontal pipe that I could cover/uncover with my thumb to make or break siphon. As the sump level went down it was able to get a siphon going, so that was good. However, once the siphon got going the flow was too much. Even with the return pump at 100% output it couldn't keep up and the sump would over fill.
Phase 4, same as Phase 3 but raised the sump. I built a little stand to raise the sump as high as the existing plumbing would allow, which was about 3.5". The PVC pipe rests on rim of sump now instead of elevated above. I also shortened the submerged discharge piping the same amount. The idea here was to get the natural levels of the display tank and sump closer together. My thought was there could still be siphon going but with levels closer together it wouldn't be as strong. And it worked! With this setup the siphon was much gentler but still giving sufficient flow. I could take the pump through it's full range of flow (30-100%), and no overfilling. That said, if the return pump shut off and siphon kept going it would still be able to overflow. So still needed a solution for that.
Phase 5, automated siphon break. The hole I covered with my thumb to make/break the siphon has now been replaced with a motor operated valve controlled by a float switch and relay. I initially wanted to use a solenoid valve as they open/close much faster. But most of the (reasonably priced) ones I found were not rated to have the coil energized for significant lengths of time. The MOV doesn't have this issue, and the approx. 3sec open/close time still worked fine for my purposes. When water level is normal and pump is running the valve is closed and allows siphon. If the water level goes high the valve opens and breaks siphon. The valve also goes open on loss of power. Keep in mind this is an "emergency" system to prevent overflow if something goes wrong, not cycling on and off during normal operation. If anyone is interested I can post more details on parts used, wiring diagram, etc.
Jeez, that got long! Thanks for sticking around if you made it this far LOL. Got too late tonight but I'll post some pics tomorrow.