My camera is Canon 6D Mk 2 (full frame). I already have Canon 24–70 mm f/4 and Tamron 70–210 mm f/4 lenses and I'm planning to get a landscape lens in the range of 10–24 mm. I want to shoot astro photos and normal landscape photos as well. I am leaning towards a f/2.8 lens due to the interest in astro photography. I liked what I saw in Tamron 15–30 mm f/2.8 but it needs external filter kit which is expensive. Canon 16–35 mm f/4 again looks decent, but f/4 is worrying me due to my interest in astro photography. Can anyone help me to sort this out? I am open to other brands out of Canon as well. I loved my Tamron 70-210mm.
The popular astrophotography lenses for full-frame cameras have typically been the 14mm Samyang/Rokinon lenses. Those are completely manual lenses. For astrophotography that's not an issue since you can't use auto-focus on stars anyway. These used to be f/2.8, but they now make an f/2.4 version as well.
Astrophotography and Landscape
For a more general purpose ultra-wide lens, Sigma makes a 14mm f/1.8 Art lens. It's a bit more expensive (around $1600 USD), but it is does have auto-focus & aperture -- and it is an f/1.8 lens. Since you plan to use the lens for daytime landscape as well as nighttime astrophotography, having auto-focus & aperture makes it a bit more versatile. (This would probably be my personal choice if I were to buy a new lens today.)
Thoughts & Consideration
The Samyang/Rokinon lenses used to have a somewhat common quality issue with "de-centered" optics (I use quotes because I suspect it may be non-orthogonal optics). In any lens, the field isn't actually "flat". There's some field curvature and that means if you achieve your best focus at the center of frame then inspect the corners, you'll notice the corners aren't quite as sharp as the center ... but each corner will be about the same as any other corner. On some Samyang/Rokinon copies, some corners were noticeably worse than others. It's been a few years and they may have (hopefully) corrected the quality issus. If you do decide to go for Samyang/Rokinon, give the lens a test when it arrives to make sure you have a good copy.
When doing astrophotography, some photographers pick a star about 1/3rd of the way into the frame to use for focus ... instead of a star in the center. This somewhat helps to average the focus quality across the frame so the corners don't look as soft (the center isn't as tack-sharp, but the corners aren't nearly so obviously soft). Also, Stopping down slightly will help dramatically with coma and astigmatism.
I would be remiss if I didn't mention tilt-shift lenses. These are lenses that are especially versatile for landscape (and architecture but you are interested in landscape). The lenses allow you to change the plane of focus so it isn't orthogonal to the optical axis of the lens. Here's why that can be helpful.
Suppose it's a rather breezy day and you'd like to capture a field of flowers. But the flowers are blowing in the wind. This means you'll need a fast shutter speed to avoid them being blurred. But you find that when you try to use a fast shutter speed, you have to open up the aperture and now the depth of field wont let you get sharp focus across the entire field from near to far.
A tilt shift lens, would let you tilt the focal plane to match the field while still letting you shoot with a wide aperture so you can use a fast shutter speed -- freezing the action and getting sharp focus from front to back with no motion blur.
These are completely manual lenses, tend to be expensive, and have a bit of a learning curve to understand and master how to use the tilt feature.
These lenses don't have any advantages for astrophotography, but since you primarily asked about a landscape lens, I wouldn't feel an answer is complete without at least mentioning them and when/why they are useful.
What kinds of filters will you be using?
The Sigma ART 14-24 f2.8 performs exceptionally for wide angle landscape astrophotography, though it can't take regular filters due to its bulbous front element.
Files captured using the Sigma are remarkably sharp, and exhibit a very acceptable level of coma distortion in the extreme corners. The lens maintains straight lines impressively well for such a wide field of view. The barrel distortion is well controlled, and can be mitigated even further using lens corrections in post-processing.
I personally compared its performance in the field to the Canon 24-70 f2.8 mark ii as well as the Canon 70-200 f2.8 mark ii, and am continually impressed by the Sigma.