If you want to join the RS3 equivalent of wallstreet bets, buy bonds and hold for the next holiday treasure hunter promo and either sell or FOMO on keys.

Many people consider luck of the dwarves a waste, but if you have Grace of the elves its not a bad combo buy for skilling.

Consider getting some decent T75/T80 armor and get the best weapons in each respective combat class you can afford with your remaining budget. (Excluding Melee, You are good with cheap spears and weapons for invention xp, the "good" melee gear costs either more then 1B combined, or requires 1B in jewelry to make it extremely effective)

At that point the rest of the money will go towards food and ammo. and you can build from there.

Don't worry about T85 or T90 and above weapons. In most cases they aren't worth your time if you don't have the money to beef them up, or to buy comparable power armor/invention perks.