Units like this definately work for planetary militia’s. Problem is they have to be spread out all over the place covering a bunch of different potential targets.
An effective invading force is going to use high value units like mechs (because ultimately moving lots of stuff in space is very expensive) with dropships. Your planetary militia might have 10,000 jeep type vehicles, 5,000 light tanks and 100 regiments of infantry but with 1,000 targets they need to defend there’s only on average a company of keeps, a lance of tanks, and a company of infantry at each one of them. Land a union full of mechs at a target, delete, then dona suborbital hop to some other place and repeat.
Sure you can concentrate forces at the really important places, but then the lower priority targets are in defended and destroyed or secured by the attackers piecemeal further degrading the plants ability to sustain and defend itself.
In order to mount a proper defense, either the planet needs to have only a few key points to defend (the spaceport, the capital city, the big factory and the mine) or they need aerospace assets of their own to concentrate and disperse forces just as fast as the attackers can (who are basically guaranteed to have aerospace lift capacity for their entire force handy, since it’s how they got there) or to massively outnumber them with reasonably high quality units so that there aren’t any weak points to pick apart.
