There are lots of bag/rack solutions. My preferred always-on-bike bag is this guy, with the supporting rack:
https://builtbyswift.com/shop/zeitgeist-bag/
Room for tools, room for large smartphones, room for stuff, can have an organizer in there. The proverbial Bag of Holding. Best of all, can be mounted on any bike with a saddle, the rack only needs 1" of saddle rail space IIRC.