I have a Topeak QR beam rack that I use with either a pannier or trunk bag (w/wo pannier).
It has the removable side frame to keep panniers out of the spokes.
I swapped out the c/f seatpost for alloy to avoid crimping problems. The original c/f seatpost had an aero profile and didn't fit anyhow.
I tried carrying a laptop in the pannier but climbing uphill rocking the bike was too much. It would be do-able if centered on the rack.
Key is packing light to avoid sway. The bottom line is that the beam rack is only good for up to about 15#. For light commuting that is fine.
I keep shoes, belts, toiletries, sweater at work; and use car days for transporting heavier/bulkier stuff.

Coffee - I carry cold coffee in a plastic water bottle and decant/reheat at work. I also have a coffeemaker in the office.