We used a Timbuk2 messenger bag and added their stork set (which was a changing pad). It’s the same company my husband uses for his work bag and he loves it. It was great. It’s been in use 7+ years and we don’t have diaper babies now but it still is in great shape. I may be about to snag it as my work bag.

https://www.timbuk2.com/pages/search-results?q=diaper&type=product

You can get a foldable changing pad and convert any bag you like to a diaper bag. We were given one like this (not our brand but close enough)

Baby Changing Pad by Lil Fox. Portable Changing Pad for Baby Diaper Bag or Changing Table Pad. One-Hand Diaper Change Pad. Baby Shower Gifts, Newborn Baby Essentials, Unisex Baby Stuff https://smile.amazon.com/dp/B07CX4SPBH/ref=cm_sw_r_cp_api_glt_i_ZQV8M3R73DVQ1AVTM5TJ?_encoding=UTF8&psc=1

The only thing I ever saw that I was jealous of in a diaper backpack was one of my friends had an external pouch for her wipes. And she could grab one quickly to wipe a face or hands without getting everything else open.