I have a set-up almost identical to yours, but on a 90gal. Sealing the lid on the sump will help a little, but you (and I) have a passive overflow that draws in air. This is different than Blazingwolfs. His is based on a siphon at the top, without drawing much air in.
You can minimize, but not eliminate CO2 loss. Its not that big of a deal. You need to up your CO2 to about 6 bubbles/sec, slowly and read your plants. Injecting CO2 is about perpetual CO2 loss; accept this and you will be happier. I spent a lot of time sweating every bubble that made it to the suface. CO2 is cheap; and a cost of doing business.
Below is a descript of my set up and how I reduce loss and improve CO2 water absorption.
My sump is almost the same as yours. I use bioballs in my chamber, with a drip plate on the top of them. I have a course filter pad on top of the drip plate to act as a prefilter. I change it every 1-2 wks. My overflow intake spills right onto the filter pad. Just like old school salt water trickle filter. I keep my sump filled as high as possible. This helps. I seal the lid with duct tape; this helps. Your overflow intake going below the water isnt't helping much, and makes prefiltration harder. The water falling through the tube is where all the CO2 loss occurs, and the top is open to air. This is the only bad part of our set up.
Like you, i keep my overflow elbow up high so water only drops about an inch. Again, CO2 is lost when the water travels down the drain tube. Nothing we can do about this with our set up; and not worth the effort.
I initially injected CO2 into my needle impellor. this pump is also my sump return. You get lots of microbubbles. I have a PVC reactor inline after the pump. It is a 3" wide 20" tall beast. It did not catch very many micro bubbles. I went extra wide hoping an eddy could occur inside, catch more bubbles; no luck. I now inject my CO2 right after the pump and before the reactor. I noticed about 60% less bubbles in the tank. my drop checker still goes from dark blue to very light green in about 2 1/2 hrs. I have good plant growth. I use a ball valve on the return to reduce the overall flow into the sump and to slow the return through the reactor. Its about 3/4 open on a 700gpm pump. this has seemed to help. I direct the outlet flow toward the bottom/ opposite corners of the tank using those Y-flex nozzles, and use a Hydor for tank circulation
If you have a good plant load, a high bio filtration is not as important regarding the volume going through the sump. My sump at 50% probably filters better than most canisters, IMO.
higher light if you want aggressive growth, less light for slower growth. I dose EI, and add a bit of excel at weekly water change.
hope this helps. give me a yell if you have any questions, or want pics of my set up.