I got a deal in a craigslist buy where I picked up a full set of Topeak MTX bags and racks and they are one of the best things I’ve bought. I have large and small bags with collapsible side Panniers and a basket all of which just slide on and lock into place on any of my MTX racks. I even have one quick release rack that attaches to the seat post and can be added or removed from my MTB as needed. I use the large rack for my commutes and can collapse or expand the side pannier sections as needed. I use the small bag when I want to go out for a ride but might need some extra storage for layers or think I might pick something up while out.

I have a separate carbon road bike so if I want to go fully light then that’s what I ride. If the weather is iffy, or I might need to carry stuff I take the aluminum/commuter/gravel bike with the rack.