Due to how the mod works - it is not possible to launch rockets.

Single line I've tried using should fix this since the silo uses a fixed recipe:
data.raw.["rocket-silo"]["rocket-silo"].fixed_quality = "legendary"

So far it has worked, but haven't yet tried to send a rocket.

Yep. This works. Easy fix for anyone that wants to use it until it's updated.
Edit the data-final-fixes.lua to include:
data.raw["rocket-silo"]["rocket-silo"].fixed_quality = "legendary"
Then it will set all the ingredients to also require legendary.

New issue however - Can't used quality Space Platform Foundation as actual space platform foundation

To anyone else reading this in the distant future - there's a lot of small things that ultimately break and essentially require you to have a dequality mod. Pretty much starting on any planet requires it due to the mix of not having an assembler and getting HQ components from smelters. If you are going to other planets be sure to send down an assembler when you land.