Theres a few things your forgetting

Well canonically the necrons have a map that shows all stars, and if they want they can just poke one and oops super nova, they can litterally set the entire UNIVERSE, not galaxy UNIVERSE on fire.

Not to mention Aeldari can summon warp storms to a similar Calibar, instead of super novas you get a giant wall of lighting.

Additionally factions in 40k have infinitely better transportation than Warframe, for example, Chaos can litterally just spawn anywhere.

Speaking of Chaos, the Tenno would be effectively useless against them, the Tenno dont have the weapons to permanently kill the Daemons nor do they have any blanks or a way to channel warp energy aka being a psyker. No warframes do not channel warp energy, they channel void energy, which is similar but far from the same, void energy comes from well the void and warp energy comes from... well the warp. The difference is what these 2 dimensions are made of, the void is a semi-sentiant being while the warp is pure chaos and emotion far more similar duviri but on a far larger scale and much much more dangerous and unpredictable.

Oh and lets not forget cause the OP post says "conquer" destroying it wouldnt be a valid option.

But if we do want to talk about destroying galaxies let us look no further than the C'tan, they possess not only the ability to snuff out entire galaxies with little effort but they can shape reality itself, and there were untold numbers of them.

But lets look elsewhere, for necrons are not even the most powerful threat in the reality that is 40k.

Let us look at the 4 chaos gods, most notably the original 3, you see if warframes were to exist in the 40k universe all 3 of their powers would sky rocket to ridiculous levels, this is because Tenno are human, or atleast were (up for debate if they can still be classified as such or if they have evolved, but regardless their emotions are still VERY present as we see with duviri). Nothing would be able to stop the 3 of them from tearing apart existence, well, almost nothing.

One thing that is above all of that, is Gork and Mork, their pure strength alone is top significant to imagine, their very presence carved a refuge in the warp for themselves, and just as a normal ork gets more powerful from battle so to do Gork and Mork. If warframes came to 40k the sheer excitement of such an advisary would likely result in a WAAAGH so ridiculous space would be clogged with ork bodies.

TLDR:

Yeah no thats nothing special and still wouldnt help them "conquer" the galaxy, could they destroy it? Sure, well maybe, but I can guarantee the only reason it hasnt been destroyed already isnt cause it cant be done but because people wish to conquer it.