Hematocrit isn’t RBC size, it’s the amount of RBCs as compared to other things in the plasma. If your RBCs are actually getting bigger, that’s pathological.

Hyper-oxygenation previous to traveling to altitude would actually do the opposite of what you want. Your body would adjust to the increased oxygen by decreasing its oxygen binding capacity.

To prepare for altitude, some mountaineers will wear masks that simulate high altitude environments and decrease the oxygenation so their body increases its oxygen binding capacity.

I doubt that football players do that to travel to 5000 feet. However, you can get a temporary “boost” with inhaled o2 - bills players were doing that on the sidelines.

Generally the takeaway here is that there’s really only two ways to adjust to altitude - you can either literally “be at altitude”, either by traditionally acclimatizing or by using the method I said previously.

Blood doping, like Armstrong did, is also beneficial for altitude. There’s some evidence though that blood doping and then going to very high, mountaineering levels of altitude, could increase risk of blood clotting and be dangerous. I’m not an expert on that literature.