One suggestion for securing your base before leaving is to make a construction tank. Tanks can be driven remotely and have equipment grid. You can put personal roboport in there and use it as your clumsy impersonation for building outside your roboport coverage from anywhere.

Once you have Bots and all things automated to build assembling machines run by bot logistic automated you can leave. You can then build everything remotely. But's i a good idea to have the base in a "healthy" state and able to provide you with some extra resources in the beginning. For that you of course need a ship/platform that can reliable travel between nauvis and the first planets. That ship can also take you back once have built a rocket silo on the new planet.

Kinda true. You'll want good defenses in place with bots set to repair before you head out, but there's also nothing stopping you from shipping all of the components to build another rocket silo on the other planet as soon as you drop.

You can only leave a plant via a rocket. So when you get to a new planet you are stuck till you make enough resources to build a silo and launch a rocket. Or you take the resources to build the silo with you.

Once you are on a new planet you aren't starting from scratch, you can always send stuff from navis and deliver them to where you are.