Hello
after 12 years i remember i bought bitcoin in 2010
is it possible to sell more than 50,000 bitcoins at once or is it better to sell direct to a private individual like grayscale or microstrategy?
Only in your dreams... Anyway, that's a ridiculous question. Selling
anything worth $1.5 billion worth is going to take some effort.
What a tragedy is to have 50000
BTC and not knowing what to do with them!

Going back to the question, if that was my situation, I would start contacting all the major exchanges for some split OTC trades. Of course, that requires some effort, but with 50000 btc available, it should be worth it.
Small dog, tall weeds.