7:20 is pretty damn good on a second try not gonna lie. The plan for this game is that you need to decide based on the quests (middle yellow menu) where you want to invest your gold, "sideways" or "upward". You either buy a new weapon/ try to get to a new tier (3 copies or 9, similar toTFT or presumable any other auto-chess game) or level a specific weapon up (the right button with the choices every 10 levels, it even glows on the 9th to signal its arrival). These are the only 2 ways to spend gold currently.
High tiers can have massive impacts on the game, but they take up a lot of resources and with more incoming weapons they'll be even harder to hit, but it's always worth it (after at least unlocking them in the Refinery [out-of-game 3rd menu]). Base levels can get you out of trouble in a short notice, but they can fall off faster as well.
Currently it's coded that the game starts to ramp up after 6 minutes and gets even harder after 8 (so players can't "win" a 15 minute run on their first try, without any outside upgrades in the second out-of-game menu) the soft cap of the game, 15 minutes is when it ramps up even harder, trying to kill the run as fast as it can, but that's intended currently.
I really appreciate you trying it out and leaving a feedback, it helps a lot (more than people think, because it suggest to me, that the layout isn't that intuitive currently and will need to fix it later). I hope you like it and will return for the new stuff I plan to implement in the next fe days/weeks.
-Cheers