Why isn't that possible?! At least I haven't seen the possibility anywhere. It's a huge gap in the market! I'd even buy stocks with 5% or more fees and pay with Bitcoin. Can't be that nobody has ever thought of making money and offering it to you?! So why not? What am I missing?
I don't know of any platform that is offering that kind of service. Wouldn't that be a lot of work for anyone that tries to do it? And moreover I don't see why you would buy Bitcoin and then be looking for stocks to buy with it. Why not buy the stocks and make payments directly from your bank? If you're looking for something stable, there are some cryptocurrencies that are backed by assets these days or you can buy stablecoins like USDT or PAX.
I do invest in gold by local shop where I never think about any electronic payment options. But I am sure about buying gold for bitcoins in next 3 to 5 years. As of now I believe it is time for saving bitcoins hence I guess it is not right to think about investing using bitcoins.