I carry a Systainer 4 packed with tools for installations. Inside are the Bosch PS20 and charger with one spare battery, 1/4" quick change drill indexes (brad point and machinist bits), two rollup canvas bags for butt chisels and a small brass mallet and a set of 6 screwdrivers (square, philipps and slot), measuring and marking tools (Shinwa bevel, small Veritas marking gauge, 6" Starrett flexible end reading rule, scratch awl, marking knife), a Veritas apron plane, diamond stone, , folding dozuki pull saw, Starrett 12" combo square, notepad and pencils, sanding block and a few half sheets of paper in various grits.
There's probably more stuff rolling around in the bottom that I haven't seen since I first set up the Systainer. All this fills up about 3/4 of the Systainer. I bring other tools (power and hand) as needed.
I use the Bosch just as you described: drilling, countersinking and driving #8 square drive screws (maybe 20 - 30 per job). It's been perfect for this work. I carry the charger and extra battery but to be honest, I don't remember having to change batteries during the course of a job. I guess I just feel better if I have the backup.
I originally bought the C12 to fill this function on site but now I leave it at the shop for work there. If I need a larger drill, then I pack the Makita Li drill and/or impact driver.
My work doesn't usually involve cabinets. Like you, it's not big work. So, the setup in the Systainer has been more than adequate for my needs.