Making a 64 pin breakout board, looking for help with best practices and any glaring design errors. First design in a few years
Generally looks ok, and welcome :) Few things:


Make sure all your pin headers are on 0.1" spacing. Even if you're not putting it on a breadboard, this will make sure any header cables will fit nicely. See, for example, the 3 little connectors toward the bottom left.
The component to the top right seems to have a trace going from a via through the ground plane to the bottom pad. This doesn't seem to be on the same net as the plane, so could be a short.
Each power pin on the MCU package must have a capacitor on it, usually 10-100 nF will do. Use as small a component size as you feel comfortable soldering. Put a 10 uF for each supply voltage where it comes into the board. This is very much rule of thumb, but should get you started!
Some of your trace bundles could be tidied up. For example, the bottom right one has the right most trace going out to the right then down. If you went down first to meet the other traces in the bundle, the ground plane can fill in to the space. The bottom left one is nice.