To play a bit of Devil’s Advocate (from a bench-top scientific standpoint I come from immunology/microbiology background—so I know enough theory to be dangerous but don’t have your depth of evolutionary understanding) doesn’t a lot of this rely on cosmic timescales? I’m sure I could easily do a web search on this, but I think there are a lot of galaxy clusters that are much older than the Milky Way. That would give the potential for many multitudes of planets that have been around much longer than Earth, which gives a lot of time for intelligence to evolve and sustain. Now, if an intelligent civilization can ever survive for that long is a different question in and of itself.
I personally have wondered if the natural, sustainable, next step in any intelligent evolution is artificial forms of intelligence. Maybe biological intelligence is just the bootloader for less squishy forms of life? Immortal silicon life sort of renders the biological limits of space travel a lot less problematic. I know that comment exceeds the scientific into the philosophical, but it’s a thought I’ve had a lot lately.