Be patient. If you only have tens of thousands of dollars to work with, I wouldn’t be concerned that you’re hitting your CPI but not hitting daily spend.
I’d be looking near term at whether there’s a look alike audience you can generate from your acquired users to improve your targeting (and hopefully lowering your CPI), or if there are sub segments of those installing your app with higher LTV which would allow you to spend higher in those cohorts.
Creative can make a huge difference, so it’s good you’re experimenting there. There’s no magic sauce to that, and sometimes the dumbest looking creative is your top performer.
Last, I’d be looking at which networks and placements are proving best LTV wise for you. Make sure you’re optimizing towards the placements with the best return and pushing those harder. If you’re running freemium, I’d be looking at ARPU per placement rather than just CPI. Only pour gas on the placements where the ARPU is exceeding the CPI consistently.
Good luck!