Why would you want to stand back before pulling the trigger?
If you can shoot a 50 BMG, you can shoot any of my wildcats just fine. As mentioned before. the higher muzzle gas volume and pressures make the muzzle brakes
work even more effectively then the standard BMG.
H-50BMG will work fine in the Maximum, you will not have 100% load density but it will still work just fine if you pay attention and know how to read chamber pressure signs in a 50 cal weapon which is nothing like a conventional rifle.
US869 may be a better choice as its slower burning then H-50BMG. The best powder will likely be VV 20N29 as it should offer the highest load density and likely best velocity as well and has been proven extremely consistant. Just more expensive then the other powders used.
If your looking to save a buck, surplus powders such as WC872 and WC860 can be made to use perfectly well and they are around 1/3 the price of commerical powder but you have to do your bench work to figure out the burn rate of your lot of powder, thats why they are cheaper, less lab testing has been done to determine exact burn rate on the surplus powders. I would never recommend pulled surplus powder, only new.
You make these big 50s out to be scary monsters, that is not the case at all. My wife shoots my 510 AM as well as I do, your overhyping the recoil produced by these chamberings. I have several 12 guage shotguns that kick MUCH more violently then any of my 510s have. My 458 Lott has MUCH more severe felt recoil with its 500 gr SP loaded to 2350 fps. Its not what your making it out to be in the least.