I learned from the YouTube vid ‘undefended pieces’ from John Bartholomew that it is important to have all your pieces covered. Not only if they are attacked, but also when not (yet) attacked.
You mind want to have an extra eye on your pawn moves that leave pieces undefended.
So like I said my tactics rating is pretty good but I can't seem to consistently beat 1300-1400 players. I have a trend of beating 1400-1500 players convincingly at times, but then lose multiple games in a row to 1250-1350 players from stupid blunders. I feel like I give more focus to games where the opponent is much higher rated than me, and I am able to win those games a decent amount of the time, but against lower rated players I think I may play too quickly or underestimate them or something. What advice do you have for me?
The tactics rating is completely unrealistic/ unrelated to playing strength. It mostly reflects persistance.
Your rapid rating of 1358 stats tell you that you don't beat a lot of 1400-1500 rated rapid players. Your average win in the last 90 days is against a 1259 player. Your average loss to a 1308 player. Your best win ever was against a 1510.
Meanwhile, you have done 3410 puzzles. You have discovered that you can take all day to solve a puzzle and it won't count against you.
Maybe the puzzles are telling you that you need more time to think. 15 10 really isn't a very long game. Maybe you should give slower games a try.