Could the Kett be more advanced/powerful than the Reapers?

No, I don't think so. Meta-wise Bioware just told that kind of story and wrote themselves into a corner with it so would want to avoid doing that for a while since they plan for several games in the Andromeda Saga. Ingame-wise they only pose a great threat since we have only a relative small number of people and no galactic infrastructure. Plus if they were Reaper-tier they wouldn't be after Remnant tech like we are.

If they are even a tenth as powerful as the reapers then andromada is doomed. the arks can't have more then ten million people and the galatic infistructure is non existint as is the factories that can mass produce ship like they could in the milky way. That said your idea that since the reapers aren't wiping everyone out every fifty thousand years or so means that they should theoretically be far more advanced then us. It may also be why they are supposedly low on resources. They have already used a ton of them up. (heard about low resources somewhere but could be wrong). If they hadn't just done a trilogy that was based on shepard fighting reapers then I would say there would be a good chance your right. Good thought though

I don't think so: while it's true they haven't faced general extinction every 50’000, they also didn’t have access to reaper tech in the form of galaxy portals, mass effect and eezo manipulation.
And while it’s true the Reapers used their tech to force predetermined evolution in the races of the Milky Way, it’s also true that, until the times for reaping comes, it’s one hell of a jump start in technology for a new civilization. Discovering FTL without this kind of jumpstart could take centuries, millennials and/or a particular grasp on physics. Even then, Milky Way civilization in the ME universe are utterly dependent on eezo, which seems to form near energetic stars /violent cosmic phenomenon...
And there is more: eezo is toxic (deadly) for anyone but biotics lore wise.
So, to develop FTL tech without Reaper Jump start (similar to the one in the Milky way of course), you would need an element only found near violent stars, where no sane person would go, especially with slower than light travel…
It’s plausible that we, the invader of Andromeda, would survive our initial years in the new galaxy thanks only to our superior tech.
Which would it make sense: we’re less than 1 million people in uncharted territory. We need something, a defining advantage that make our survival possible.

Just my two cents on the matter.

Aside from the Kett, we haven't seen much of the Remnant...other than their robot defense drones. The Remnant could play a similar role to the Protheans and perhaps be the most close to the Reapers if the Kett aren't already. Wonder why would they go extinct/missing then...

Why are they a threat to milky way natives? Simple. Numbers.

Our motley crew numbers in hundreds of thousands, kett probably number in billions. Hows that looking now?

depends on how advanced they are....

25 hits from an Everest class dreadnought ship's mass effect cannon on their home world taking out their 25 most populated cities can be achieved in 2 minutes and 5 seconds decimating their race assuming they do not have comparable ways of fighting our kind of warfare...that is assuming we have any qualms about simply diverting an asteroid and aiming to their home planet.

Remember....the Sangheili in Halo TRIED to take on the Legkolo and failed because on the field there was no way in hell even they could outfight the hive forms in those numbers. But the Legkolo had no choice but yield when faced with the thread of orbital bombardment.

If the Kett were millions upon millions of years old, I'm sure they would've come up with technology that isn't all that analogous to the Milky Way crew, let alone our own. They'd look like they had magic or something versus aliens with guns.

I assume that they're on par or slightly above the MW peeps, but their sheer numbers has them in a superior position.

Remnants can be MW species from a cycle long time ago, whose legacy is a key for understanding the myseries of Andromeda galaxy. Ah, yes reapers  SOMETHING is coming.   

Get prepared to play the god damned underdog. Let's see, you have 4 arks that have 20K people on them, and the Nexus which let's say contains 250K people. So, that's 330K people. Against billions.

So, yah.

I don't think it's unreasonable to assume that a native, spacefaring race has huge numbers.

I'd say Bioware will just go the easier path and make their technology equivalent to the Milk Way's, even if a bit different.

It might be interesting if they go the other route and have the Milky Way colonists as slightly more advanced technologically, but at a disadvantage because of much fewer numbers, being in unfamiliar territory, and without established worlds.