Mini is going to be your simplest option I think yeah... but you'd need 4 core wire like you said or use 2 core but you'll then need three ends to your cable: 1 pair to the plug, 1 pair to the lamp and 1 pair to your little switch.Thankfully, with them being lamps, the "state" of the physical switch does not bother me (it would if I had to use Sonoff/Shelly for lights in conjunction with existing wall switches which I don't). It seems that going with @calypso's idea with using 4-core cable, but placing the Sonoff Mini out of sight perhaps stuck on top of the 3-pin plug itself could work thus retaining the existing switch as well. An option to consider.
I'd recommend getting a little enclosure box from communica or similar with small glands to keep it safe and neat.