Come on, we know for a fact from the graphic after the Suicide Mission in ME2 that the reapers flew over 150,000 LY without discharging or stopping at your local Chevron station for fuel. I think they suck up dark matter and use that for fuel. Yeah! That's what they do.
And whose to say "there's a lot here that got squandered?" Liara? She still thought those drawings in the Temple of Athame were of Asari even though Ashley could figure out they were Prothean. Wow that was entertaining. Would it occur to her that her own people didn't tell her everything? By the Goddess she was shocked that they kept a secret. What technological wonders did the Asari glean from that Prothean archive? Cryo tech that can preserve a race for 50,000 years?
And that research facility on Ilos? Oh, they told Shepard it was dead. Hmmm.... They also said "Ah, yes, 'reapers'...... we've dismissed that myth." Even though Sovereign as a REAPER was fully documented in the Citadel Archives. So whose to say the research facility was really completely dead? Bring in some power generators and extract some scientific data, and voila! more tech!
Then like I've said... the derelict reaper and Cerberus.... and that Asari Matriarch.... and The Illusive Man.... did she meld with him? I wonder. See Reaper FTL schematics, Reaper Power Core,
The details for the plot of how the Ark gets built and how we get to Andromeda are in the plot holes of the ME trilogy.