To add another element to the discussion, would the Reapers consider a species like the Tyranids to be advanced in the same way that the view the Asari and Humans advanced? What is their what is their minimum threshold for a species to be "ripe" for the harvest? Is it the ability to achieve spaceflight, the capacity to activate the Mass Relays, the discovery of the Citadel?
Technically, it is just spaceflight. However, the Reapers can only know more easily if you've achieved spaceflight and become spacefaring if you have activated the Mass Relay in your system. This is recorded by the Relays so the Reapers know quickly. If you've reached the Citadel (which would be very quickly after discovering the Mass Relays anyway), then all the easier.
Hypothetically, we've be ripe for harvest decades ago IRL-time. But the Reapers may not know unless they checked our system, and then they still have to make a judgement call most likely (one trip to Luna/Moon may not count). Otherwise, we got many many years left.
The Catalyst says that the Reapers leaves the lesser evolved species alone, but what is it's definition of evolved? For all intents and purposes the Tyranids are just a swarm of of alien locusts, sure they have intelligence, but they have nothing even close to what we would consider to be a civilization. What would be the requirements that the Tyranids would have to meet before the Reapers would consider them to be evolved enough to harvest?
Likely theoretically capable of feats like creating AI, using the Mass Relays, or the equivalent. IMO they would count.
Is it their potential to kill a Reaper? If that is the case then Thresher Maws should be on the list of harvest-able species since one took out a destroyer single handily. What about the Varren, if races like the Krogan are using them as war hounds to attack husks, does that make the Varren targets for assimilation?
Is it there potential to kill a Reaper? ABSOLUTELY.
Thresher Maws couldn't be shown to kill a Reaper as a species until now, and it took the Mother of them to do it. IMO if once Destroyer was killed by a Maw, the Thresher Maws may have to watch their asses in the current/next cycle (in a Refuse ending). I also wouldn't say it was 'easy' for the Maw, only quick.
Though I'm not sure if they would be harvested or only killed off. Hell, for all we know, maybe the Maws are culled every cycle, but never eliminated due to their proliferating nature.
Varren are nothing. I hope that wasn't serious.