More Words, Deeper Hole

I really like how House of Suns by Alastair Reynolds takes on what you called "Phoenix cycles". House of Suns is set six million years from now, where not only the entire galaxy has been colonized (all at sublight speeds, BTW), but almost every inhabited planet has had five or six apocalypses. What protagonists call "microwars" (i.e. wars that engulf only a handful of inhabited systems) and subsequent re-colonizations come with such regularity, the general term for it is "turnover". And nearly every inhabitant of the galaxy has some relics of some bygone civilization to look at -- usually several.

These planets rarely regress to complete barbarism thanks to Lines -- societies which completely decoupled from any kind of fixed base, and spend their time traveling the galaxy at near-light speeds. So almost every inhabited planet gets visited by Lines every few millenia or so. Enough to maintain at least modicum of knowledge.