I’m going to do the old favourite of recommending what I have, and that might not be completely relevant to your situation. But it is something different to posting another vote for the expensive bags above.
I love my decathlon pannier messenger, like this:
https://www.decathlon.co.uk/900-cycling-messenger-bag-black-15l-id_8385332.html
I had a look at similar options from Vaude and Ortlieb, but they all seemed compromised to me. I like the fact that the decathlon bag has a flap to hide the rack clips. I was worried that it didn’t seem like a secure attachment system, but it is very quick and easy to lift off the rack, and I rarely use the securing toggle. I have never had it come off, even on off road jaunts. The bag isn’t huge, but it is big enough for a commute with a laptop and a change of clothes. The low profile also avoids interference with my large feet. I have also done a lightweight hostel staying tour using only this one bag. I’ve toyed with the idea of getting a 2nd bag, just cos my one is so good. It’s not completely waterproof, but my commute isn’t long anyway so the shower cover is light and effective, and it is quick.and easy to open the bag. The price is reflected in the quality of finishing and the materials, but it is sufficiently sturdy and smart looking.