Reddit mentions of Topeak Super Tourist Tubular Bicycle Trunk Rack DX with Side Bar

For people using rear panniers, I'd definitely recommend they look at the Topeak Super Tourist rack - it has the same mounting system as the Explorer model, plus it has an extra set of side bars so (1) you can easily use panniers and a trunk bag at the same time, and (2) the panniers are mounted lower and further back on the bike, which improves stability and helps avoid heelstrike. I've used the Topeak Super Tourist rack with my Ortlieb panniers for daily commutes and long-distance touring and it's worked great.

I'm also a big fan of the Topeak trunk bags - the Quick-Track mounting system used by the Topeak racks and bags is super-easy to use and very secure.

I use the DXP for my groceries every week, and have for years. The biggest issue is that when putting stuff into the panniers, you may have to take stuff out of the shopping bags and let them be loose in the panniers to get optimal fill. I'm not sure if I've put 20 pounds in each bag, but I've definitely carried 20+ between the bags. I've carried two twelve-packs in each pannier several times, and the weight was a non-issue.

But I also wanted to be able to carry more groceries, so I ordered those exact bags from Bike Bling. Sadly, they sold out and but were super-friendly and gave me a free upgrade to the Soma Fillmores, which are practically identical. They work great with reusable grocery bags for carrying stuff, and with my Tourist rack, I can use my trunkbag AND two grocery panniers! The main compartment of the DXP is insulated, so I try to put my cold stuff in there, and it's never been an issue. However, you will need one of the Tourist racks that has the second set of rails for panniers to use at the same time as any of the Topeak trunkbags, so be aware of that.

I also have the TrolleyTote that /u/tepidviolet recommended. It definitely holds more than the main compartment of the DXP, but it puts the weight up higher, so the bike feels wobbly if you're getting 20+ pounds of stuff. I love it, but I'm not sure I'd recommend it for your solution. But the nice part about Topeak's MTX system is you can swap all the different parts out! I even have an old Office Bag that I can put on if needed, but I think it's discontinued.

So, if you want my recommendation to get the best bang for your buck: start with one of the various Topeak Super Tourist racks for your bike and get the MTX Trunkbag DXP. I use this bag daily for commute, groceries, errands, etc. It holds a good amount of stuff, and you can pack up/unload the pannier sides as needed. With the Super Tourist rack, you'll have the ability to add on or swap panniers as needed while keeping your DXP, to increase or specialize your load.