For dive 1, I would use 32% as others have stated above, the 30 min BT will *probably* be within NDL and any deco should clear on the way up if you have spent any time on shallower parts. I would then carry a "deco" bottle of 32%, which would allow the training benefits of switching bottles etc but without the risk of actual deco on a dive that doesn't need it. You could switch back to backgas once the drills are completed, leaving the 32% deco gas basically unused if that made financial sense. Going into deco when it isn't necessary makes my risk-tolerance itch....
For the second dive, top up your 32 leftovers with air and plan the dive based on what's left, should save a few bucks since its an air fill at that stage and its nowhere near as frustrating as listening to your unused helium draining in a fill room.
Your certification of "one deco gas, limited deco" is a common one throughout the agencies. The idea is for limited exposures and less complication before moving on to multi deco gases, so the fallback is backgas deco if you lose a gas. In the dives I have run/planned in that category, 50% almost always comes out tops due to logistics, standardisation and not much deco penalty. Thats not to say that a team can't use some gas that makes sense but there are few reasonable arguments for something like 65%.
My thought would be to stick with the gases everyone is most likely to be using throughout your tech career and suck up the few minutes of extra deco unless those few minutes are a major issue (serious cold water etc etc).
Also, keep an eye on those gas densities, you REALLY don't want to be doing ANY serious effort on the bottom at 6.5g/L .That can bite you hard and fast especially if its cold and/or dark. I had a benign dive go a little sideways with some current recently and boy did I wish I had some helium in the mix. To the extent where I (personal choice) am not diving nitrox deeper than 130' again.
