10 speed rear derailleur only shifting 7 gears....losing my mind
I believe your problem is a mismatch of cable pull ratios between the shifter and derailleur.

The rear derailleur actuation ratio is how far the shifter cage moves for a unit length of cable pulled. The amount of cable a shifter needs to pull for each gear shift is determined by the actuation ratio and the spacing between the cassette sprockets. 

Generally, Shimano road 10 speeds and below and mountain 9 speeds and below used the same pull ratio (the exception being Tiagra 4700). Road 11 speed  and MTB 10 and 11 speeds require more cable to be pulled for each gear shift as it makes it easier to get the accuracy required out of the shifters to index more closely spaced sprockets.

Because you have a 10 speed MTB shifter it's mismatched with the road derailleur. Presumably you got the MTB derailleur because you wanted to be able to run a wide range cassette, but you are using a road cassette anyway. Simplest fix is to get any Shimano 10 or MTB 9 speed derailleur (but not Tiagra 4700) that can accommodate the largest cassette sprocket you want to run.