Yes, that looks like there’s room for improvement.
For this drone it’s probably too late, but for your next drone keep this in mind: Hardware To Avoid When Building A Multirotor (and I don’t mean “take it as a shopping list”)
Also have a look at the How to methodically tune (almost) any multicopter guide, for now especially “Chapter 1: Getting the hardware right”. Having the battery so far in the back puts the CG in a really awkward spot where your rear two motors are doing 80% of the work.
After you corrected that, follow the rest of the guide for the tuning.
Regarding inverted RC channels: In Mission Planner in the RC Calibration screen, there are little reverse-checkboxes next to channels 1 to 4. If you tick (or untick) those, the respective channel changes direction. Also explained here: Radio Control Calibration.