Meanwhile, I found a manual but pretty painless solution. This works even remotely via construction bots.
Disable auto-launching rockets
Place a requester chest next to the silo (but no inserter)
Set the chest to request all the things you want to send to the platform. An easy way to do this ...

Thanks for making this post, found it wondering if I was doing something wrong.

This is not exactly trivial because: consider a naive solution. Sort requested items by weight, put in heaviest item that fits, repeat while there's anything that can be added. Now, the problem is: more likely than not ...