If you are having high Ethylene problems, you can use a photocatalytic oxidation device. I Design plenty of CEA rooms and we usually go with the customers request. But we try not to get the rooms to 100% sealed. A little bit of breathing and enough scrubbing makes it so there is no smell, and you don't get too much Ethylene build up. But we've had customers get over 4 LB a light using the rooms we built. But usually you have to use VPD methods, and co2 tanks, or co2 bags. Strains can usually be tackled by raising or lowering the lights for those strains. More indica usually like lights further away, while sativa likes the closer light. If you can't raise or lower your lights, you need to train and plan for where you want your plants to stop growing into flower. However, we do use timers for fresh air, as the more fresh air, the more intense your cooling/humidity/co2 application has to be.
Also I would disagree an RTU is more efficient than a mini-split with variable speeds in anyway whatsoever. Hepa filters would also have to be installed after the unit. Which causes the filter to usually be clogged faster than just recirculating, and HEPA filters aren't so cheap, especially with needed usually sizes exceeding 25"x25"x2" Those are usually $150+ a piece and you'd be replacing them every 2-3 months with outside air, otherwise you'll starve your A/C of air and you'll have problems. Plus ducting is usually not necessary and we try to never use it.
This is why we usually recommend timed fresh air, with HEPA and mini splits. If that's what the customer wants. But either way, we see substantial yield increases in both styles and I haven't seen a lot of the types of problems you've explained, but maybe we're lucky?
