Is there a reason to go with DSLR instead?
There might be a slight one, depending on exactly what type of astrophotography you're planning on doing.
The sensor is on and energized the entire time a mirrorless cameras is turned on. This creates heat that can affect the amount of camera generated "read noise."
A DSLR's sensor is energized only when actually taking a photo (assuming one is not using Live View, which is functionally the same as using a mirrorless camera as far as astro work is concerned).
If you're taking very long exposures or long sequences of shorter exposures then there's not much difference. If you are taking fewer exposures separated buy longer periods of time in between them, there could be an advantage to using a DSLR instead of a mirrorless camera.
As with all heat related issues, there will be a greater impact in warmer environments than in very frigid environments.
Is there a huge advantage for full frame sensors over APS-C?
It all depends on how you define "huge."
Are FF sensors ten times better than APS-C sensors, or even Micro Four-Thirds sensors? No.
But FF sensors do collect 2.25-2.5 X more light than an APS-C sensors, and 4X as much light as a Micro Four-Thirds sensor.
That's pretty significant. It's an over one-stop advantage over APS-C and a two-stop advantage over Micro Four-Thirds.
Given that it's my first camera and will have a learning curve, are there any recommendations to go with a particular brand?
Not necessarily particular brands, but do pay attention to cameras with sensors that do heavy handed on-die NR that are known as "star eaters." Some (but not all) sensors made by Sony for their own cameras as well as some Nikon models have been labeled as "star eaters". Sensors that do less NR to the analog information coming off the sensor may look noisier than their counterparts that do more, but they don't mistake dim stars for noise and eliminate them before analog-to-digital conversion.
I've been inclining towards sony a 7 II or sony a 6400. Should I worry about the e-mount (if i want to upgrade the camera a couple of years later)?
I wouldn't necessarily worry about the E-mount at this point. If you decide to upgrade later to the point that the throat diameter of the lens mount makes a significant difference, you're going to be spending a lot more on lenses than what you're considering spending for your first body. So changing systems at that point won't have a huge impact on the final cost.
The throat diameter of the E-mount won't be a consideration if you plan to couple your camera with a telescope via a T-mount adapter that will have a narrower throat than your camera.
What I would worry about with the α7II is that it's one of the worst offenders as a "star eater", and of course the α6400 has an APS-C sensor.
What else have I missed?
Astrophotography is all about lenses. You haven't even mentioned what lenses you are considering. You've also not mentioned exactly what kind of astro work you want to do. There's a big difference in the types of lenses you'll need to do wide angle "milky way" or "star trails" types of photos and deep sky imaging of dim Messier objects (where you'll probably be using a telescope for your lens). Whether or not you'll need some type of tracking mount will also depend on what your intentions are.