Being a level 21, you've spent some time in online, is that what you're experiencing when you play?
If it takes someone using explosives (which are limited) and chugging tonics to beat you then that's a win for you. As those items take some effort to acquire.
Also, as I'm sure you know by playing to level 21, this isn't a battle Royale, where the last one standing "wins" . There is no "winning" against other players out in free roam. You may get into skirmishes. But how long you decide to fight is up to you.
You just roam the map making money and getting into adventures. If you decide to stay and fight a player that's your decision. You can just as easily ride off (defensive) if you come across an explosive using "tonic monster".
No, you can't buy your way into dominance in free roam. If you can free aim headshots, you will beat an auto aiming tonic chugger 90% of the time.
Full disclosure: I'm too old and blind to free aim well. I count on auto aim to get me pointed in the right direction quickly. And someone who is good at free aim will clean my clock. (I don't use tonics in free roam bickering though, waste of money).