I finally figured out how to beat this game! Or at least as much as you can beat any endless game. Turns out, the extra ammo is a poor choice because it increases reload times too much after a while even if you try to be smart about when you reload , and that you ideally need a balance of attack and defense stats especially in the early game. Here's what I managed to get before I eventually died anyway.
You survived 26.4 minutes and killed 629 enemies
I also got every single upgrade! Eventually, I had no rest stops LOL.
It's still kind of tricky to tell exactly when a monster is in range before they attack, but if you can reload faster, it's not as much of an issue as long as you stay calm and make sure you are prioritizing the best targets. Knowing how far to run before resting is also tricky. You don't want a bunch of monsters to pile up behind you and to make your self too weak to attacks, but you also don't want to be constantly fighting without getting more upgrades. I think the whole [running reduces health] aspect of this game is really interesting!