It seems folks recommend dumping the trub first, but I tend to want to rack it fairly soon after that due to oxidation risk.
You're overthinking it a little.
The first trub drop should be at high krausen (about 8 to 12 hours in) to remove break and spent yeast that's settled. Cold break will probably be floating, so you won't get much.
The second trub drop should be about four days into fermentation. Just the heavy peanut buttery stuff. Once it goes a little loose, that's harvestable. Wait until 24 hrs after primary has stopped and then harvest. That's the workhorse yeast.
In that time frame, the final stages of fermentation are happening and there is still a lot of CO2 coming out of the beer. It'll displace any of the minor amount of air that comes into the fermenter. The air volume is equal to what you remove, so it really isn't much.
After harvest and fermentation, if want to remove more yeast, then I'd suggest just using a CO2 hose at the airlock port to put a pound or two of flow into the fermenter while you drop more yeast. As long as CO2 is lightly flowing out of the airlock hole, nothing is falling in, including air.
In all of these, the flow should be pretty slow and controlled. A lot of yeast and trub stick to the sides of the cone and you'll pull a hole through it all if you go too fast. It's the same friction effect you see when milling grain where the center falls first and fastest.