I use the Arena Fastpack 2.1, it's great, I can carry all the stuff I need for a day, including kit for two sessions, food and then my laptop and paper for University as well, plus it's fairly cheap, at least it was when I got it from Wiggle

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Disclaimer: Not a triathlete anymore, just a fish

Not a dedicated triathlon bag, but check out the Osprey Transporter 40. Might be up your alley, (real) lifetime warranty.

https://www.ospreypacks.com/us/en/product/transporter-40-TRNSPTR40.html

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JustinDoesTriathlon

Owner, FuelRodz Endurance.

I have the Ogio Endurance 9.0 and as a transition bag it's awesome. I also love traveling with it as the compartments are awesome to organize and separate show compartment is nice for car camping trips, etc.

For walking around transition or through airport the back pack function is great. As a backpack for commuting I'd avoid it. It's really a duffle bag you can strap to your back. But it wouldn't be comfortable commuting on a bike or train.

I'd get a dedicated "real" backpack. For commuting to/from work I have the Camelbak Fourteener 24. Enough compartments for clothes, lunch, etc.

I'd go to a local REI (or similar) and check out all the options. I think a real backpack that fits your needs is the way to go. The Osprey recommendation was also on point. I actually waffled between the Camelbak and an Osprey model.