I used to commute by bike to school. It was a shorter distance, but I had a pretty heavy load - a HEAVY IBM laptop, several thick textbooks, a couple of binders, etc. I’d say I was carrying 25-35lbs easily. Often more. I started out using backpacks, and then went through several messenger bags - finally ending up with a Chrome. The Chrome was the best of the bunch, by far - but the two biggest problems when carrying a heavy load are all that weight on one shoulder (more of an issue when walking than when actually riding), and all of that weight on your taint.
Messenger bags were designed for quick access - they’re super fast to whip around to the front, pull out a package, and then toss back on the back for the short blast to the next destination. With the stabilizing strap that most of the better bags have, longer trips and harder efforts are reasonably comfortable - as long as you keep the weight within a sane range. (I never wore bike shorts for the commute either, which didn’t help the saddle comfort/pressure thing). If your pack load is soft it will sit on your back better than if it is rigid.
A rack and a set of panniers will be by far your best bet. Bar none. They’re nowhere near as “cool” as a messenger bag, and are less handy off the bike (though Ortleib does make a cook backpack system to carry their panniers) - but for your actual commute it’s a FAR better system.