I'm currently in the same shoes as you, CaptainSlow. So no recommendation yet, but I'd share my research. I need to carry a 14 inch laptop, shoes, gym clothes plus the occassional few items of groceries my wife randomly asks me to pick up on the way home. I'd prefer the bag to have some sort of strap (shoulder or backpack), because I wouldn't leave it on the bike and will have my hands full with helmet, gloves, knee protectors when I get to the office. I don't care for price if the product works well. Currently considering:
SW Motech Sport EVO:
Capacity 14-21 liters (expandable) and pretty wide bottom to accomodate a laptop. It has a proprietary locking mechanism for easy take on and off and it doesn't touch the tank. However, the maximum load designated by the manufacturer is 3kg, hardly enough.
You can attach it as a tank bag using this: https://kriega.com/sports/touring/tank
Supposedly great quality, and the brick shape makes it easy to pack. However I'm not sure about the tank-bag conversion kit, as you need to remove the whole thing for refuelling. Plus you need to attach/detach all four straps each time taking the bag on or off. Can be used as tail bag as well.
Givi UT809 TanklockED
Seemingly does everything the Alpinestars tankbag does, but not expandable (20l capacity). Has a proprietary attachment mechanism that is screwed onto the tank opening similarly the the SW Motech bag so easy on/off. It can be locked to the bike with the key, but I'm not sure about that as the zippers can still be opened. As its weigth is supported by the console screwed onto the tank I'm sure there's a weight limit here, but the manufacturer's site is currently unavailable.
I'm also considering the Alpinestars one, but it's alarming that there are no mounting instructions or meaningful reviews of the product. The local Alpinestars dealer does not carry it either...