I think you'll be happy with that purchase. I was going to recommend at least the Topeak MTX rack, because you can hang traditional pannier bags from it (or strap a minimalist trunk bag to the top) like any other rack...but you can also use the slide-in MTX bags if you want to, which really are nice. I have the same setup (rack and DXP bag) and, though I haven't "toured" with it yet, I have carried a lot of stuff in there. We did do about 15 miles around the Tidal Basin area in DC on peak cherry blossom weekend earlier this year (2019). I had our picnic lunch (for 8) in the main compartment, and bottles of water (for 8) in each of the zip-out panniers.
Another tip: I also keep two
CVS First Aid Kit case bags in there. The website doesn't show the inside, but they zip open and have some pockets inside. You buy them empty for 4 bucks and fill them yourself. I use one for medical supplies (including bug spray, sun block, Neosporin, bandages, etc.) and then I have the other one for bike tools (CO2 cartridges, multi tool, bead jack, tire scabs, etc.). This way, the kit bags are portable and keep all my stuff organized. And I can move them around and put them at the bottom of a pannier bag if I want to use the main compartment for something else.