Try to avoid selling guns until you buy all the upgrades. Gun prices are based on your income. The more money you make per second, the higher the gun prices are.
Shoot multipliers were heavily nerfed because of the additional drug upgrades. Before the nerf it would have taken over a week for my next rank(200B exp). After it, it took me only a few days to hit 1T exp.
Part of the shoot income comes from the drugs due to the prestige shoot upgrades, so it might look like you're not getting full value from the shoot reward upgrade.
Nice suggestion