A table saw is probably the hardest way to go for cutting very close miters. Saying that, I have found using the miter gauge is much more accurate than trying to tilt the blade to 45 degrees and pushing wood through that way. A couple quick ways to fix what you already have. Take the thinnest section of frame, front to back, and set the fence on the saw to that thickness. Then carefully run all 4 sides through the saw. That will take care of the front looking uniform in thickness.
The corners will depend on how tight you want them to be. If the miters on the front are tight enough to make you happy, and you are fighting with side edges that overlap, or protrude beyond the sides, sand them flat and consider running a router design around the outside edge. That will take the attention off the outside corners. Or start over and call it a learning experience. Remember if it's going in a church, do all work as unto the Lord... [Non-text portions of this message have been removed]
