My automower have been opened too many times/with too much force, so the thread is gone some places.
Could I use some building silicone I'm the holes, or will it come out the next time I unscrew?
I think you're right that using silicone or some other filling compound would just come out after unscrewing a few times. Personally I'd look at replacing what I assume was a self-tapping screw with a metal thread screw and use a threaded insert such as the following:
So just drill out the hole a bit for a tight fit of the insert and add a little epoxy to hold it in place. Alternatively you could try to do a similar arrangement using a standard nut by drilling to a fairly shallow depth at the bottom at a diameter just large enough for the nut and using epoxy to hold it in place.
Filling with jb weld or another epoxy and drilling a new hole may be the easy way to go, but a heli coil will also work just fine, I use the standard metal heli coil inserts all the time in plastic.