>The first and most obvious is that even a long-lived civilization could construct a technology allowing a non-trivial amount of mass to accelerate to 0.1c and more importantly decelerate at the destination to a relative velocity of zero to facilitate the colonization.

Is there any reason to believe this should be impossible, in principle?

Note my use of the word "impossible", as opposed to "extremely difficult". The colonization timeline is still the same order of magnitude if it takes 100,000 years of research and engineering to crack the problem. Think about what humanity has achieved in the past 50 years, then multiply by 2000.

It’s unknown.

It can equally be possible, impossible or not worth it.

Interstellar rocks crashing at you with velocity of 0.1c might hurt a lot.

I would not like my government spending 99% of everybodies income for 100 generations, just to send one human to proxima centauri.

Growth and efficiency gains are not guaranteed, and will eventually stop. (If you take the mass of universe, put it into mc^2, and assume 5% energy consumption increase per year you get 2k years to consume whole universe worth of energy)

We can’t just assume that humans will reach Proxima Centauri.