Your doing a fine job & saving a fortune. Been doing this stuff for 35 years never seen a square house. Worst house I ever trimmed was the most expensive home 1.4 million 20 years ago. I hung a bank of mahogany wall cabinets about 14' long. You could slide your fingers between the back of the cabinets & wall. Buying your cabinets the way you did you can tweak them to look three times the price. Since you have oak fronts think about a transparent stain so the grain peeps through a little. Hardware can make a cabinet pop too. Working on other peoples homes for a living then coming home to work on mine was always hard for me. Drywall is second worse to painting for me. Your rewards will be well worth your efforts when your done.