Give it a limited job
Lay out your phone, wallet and the documents you actually need. Add only things you expect to retrieve regularly while travelling. If you also want to carry a large water bottle, camera and rain jacket, a daypack may be more suitable. An overfilled small bag presses against the body, becomes difficult to close and loses the advantage of quick access.
Check the inside measurements using a document cover or the largest intended object. A zip may restrict the usable opening more than the outside width suggests. You should not need to bend important papers to push them through the entrance. Also check whether a particular pocket can be reached without opening the entire bag and exposing everything inside.
Try the strap with your clothing
Wear the model over the jacket you expect to use on the trip. A strap that fits over a thin shirt may become too short over several layers. Check that the adjustment stays in place and that buckles do not sit somewhere uncomfortable. The bag should not continually swing across your body during normal walking.
Sit down while wearing it, then remove it while also wearing a backpack. Tangled straps create unnecessary effort, especially in narrow train compartments. Work out which bag goes on first and how both can be put down. A broad strap may distribute weight more comfortably, but cannot automatically make an overloaded bag pleasant to carry.
Inspect closures and seams. A zippered bag only remains closed if you actually use the zip. Claims such as theft-proof should not create false certainty. Keep the contents manageable, remain aware of the bag and consider the surroundings. Additional security features do not replace deliberate handling or guarantee that belongings cannot be lost.
Pack for the transition into the city
Separate things needed during transport from those for the later city visit. After arriving, you can leave unnecessary paperwork or charging accessories in your accommodation. This keeps the smaller bag focused on its purpose. An empty bag that folds flat inside your main luggage can offer flexibility when baggage conditions require it to be packed away.
Before flying, check whether the crossbody bag counts as an additional piece or must fit within your permitted item. That depends on the particular booking. Rehearse the latter situation at home with your main luggage already packed. The arrangement should still work if you really have to put the smaller bag away before boarding, rather than depending on it being overlooked.
