Register the drone. It takes 10 minutes and $5. What would make you hesitate?
In addition to the strobe you may find you'll want to add leg extensions and perhaps a lens hood or and ND filter, plus some velcro. IMO, with the Mini and
Mini 2 leg extensions are essential as you cannot launch from even the shortest of grass or weeds or land in grass without turning your Mini into a weed whacker. So don't worry about registration and you won't have to worry about breaching 250 grams. My
Mini 2 and formerly Mini1 ran (and runs) closer to 300 grams. IMO the 250 gram cutoff is silly as the extra 50 grams/1.5 ounces can make the drone safer.
As far as use of the
Lume Cube (or other strobe) I suggest you figure out a way to mount it on the bottom (without blocking the vents) and use it even in daylight. The strobe will increase your ability to see it from 400' (which is nothing) to 1600' so you can stay compliant with VLOS at longer distances.