So yes, your title mentioned "interplanetary" civilizations, but the paragraph description mentions "inter-stellar" civilizations, which would be VERY different types of civilizations.

I'll go ahead and assume you mean mostly "interstellar" civilizations, and focus on that, as that is indeed a very interesting and stimulating question you ask!

But... herein, is perhaps a flaw in your argument... maybe:

"organic beings are far too fragile and perishable to make the long journeys between solar systems."

I actually see things as quite the opposite. If anything, organic beings have proven quite the opposite and quite resilient over the course of Earth's history.

To quote Jeff Goldblum: "Life finds a way!"

But ya, ignoring annoying Hollywood quotes... I actually believe with CURRENT technological levels we could already be launching several multiple INTERSTELLAR missions right now, with current technology, if we REALLY wanted to, as follows:

1) Launch several missions to an asteroid, utilizing the cheapest possible launch-vehicles available (maybe something by Space-X?).

2) Inflate a giant clear-transparent bag (tent) around the asteroid, filling it with a nitrogen/oxygen atmosphere.

You could then, in theory, float around between the asteroid and outer tent-bag walls, wearing nothing but shorts and a t-shirt!

Hmm... Maybe inflate a couple for extra bag-tents around the asteroid for some extra protection!

3) Begin digging tunnels and carving out open spaces inside this asteroid.

We already have quite advanced mass-tunneling machines, and understand that engineering process very well.

4) Hopefully, we should also sell a portion of valuable metals/minerals dug out from the asteroid, back to Earth, to offset some of the cost of the mission, and trade it for biomass, as we shall see below.

5) Cap off the tunnels, and begin building a somewhat vast city within that large cavernous open space you have hollowed out inside the asteroid.

6) Begin spinning the asteroid, until you generate 1G of centripetal force, or something close 1G at any rate.

7) Begin filling the interior with living biomass: oxygen producing algae, animals, plants, farms, insects, along with supporting minerals, and lots and lots of water, forming lakes, etc...

All the animals and lakes and forests would "stick" to the outer asteroid wall, once it was set spinning.

8) Utilize SEVERAL nuclear generators/powerplants, to power lighting, and farming grow-lamps inside.

We're talking about a LOT of nuclear generators! This energy source has to last a few generations.

9) Move out to the stars!

And be sure to cross your motherf-cking fingers, on a motherf-cking asteroid, hoping that nothing serious goes wrong!

10)

INTERESTING NOTE: The thick rocky walls would insulate you from much interstellar high-energy/radiation particles.

In fact there may come a time, in which human life goes extinct on Earth, and the only remaining humans and large mammals are the ones inside these spinning asteroids, on their way to the stars!

11) But of course, as you pointed out... make no mistake: life is also sometimes momentarily fragile before it can evolve and adjustment/solution to new environments and problems.

And thus MANY of these so called asteroid arks would fail.

But a few just might make it after several generations.

NOTE: if you want to read an amazing book on an interstellar mission, I recommend Kim Stanley Robinson's latest book:

Aurora.

Interestingly, that novel, Aurora would actually be an argument mostly in your favor: that there are too many delicate balances that can be thrown out of whack, and missing/depleted micronutrients, inside such an enclosed small ecosystem, that could go wrong with these arks, on their insanely long journeys to the stars.

But nevertheless, the book also illustrates how it just might be possible and doable. Maybe.

And of course it would be nice to have some propulsion to push the asteroid towards the stars, and a high speed... something like exploding nuclear bombs behind a vast metal plate attached to the asteroid... or laser-sail, etc...

But it's theoretically doable. And so that just might be exactly how such an interstellar civilization looks like... to answer your question.

Sign me up!