Samyang (and Rokinon, Bower) lenses are terrific value, and some of them are at the very top, compared to other objectives available in PK mount.
I own four of them, 85mm, 35mm, 24mm and 14mm.
The first and the last ones are very good, the others are extremely good. Especially the 1.4/35mm.
From what I read the 2/135mm is as good as the Zeiss, IQ wise. Build quality is inferior, but I think we can't expect first class mechanical quality from a lens hugely cheaper.
I never checked the MTF charts of the 20mm. From the performance of the other Samyang lenses I use I expect a better performance than all the other 20mm primes you can find second-hand.
Three out of four of my Samyangs are almost always in my bag.
Maybe one day I will also buy the 20mm
For the moment, I'm fine with the Pentax-A 2.8/20mm and the Vivitar/Cosina 3.8/19mm PKA (which in reality is a 20mm) for full frame, and the Sigma 10-20mm for APS-C. Long time ago I had a CZJ Flektogon 2.8/20mm, sold it because the Pentax is much more practical. I regret that choice, now it would be useful as a cheaper alternative when I am concerned about possible theft or damage.
I also owned the Pentax-M 20mm. Great travel lens because of its diminutive size. I guess it should be more than fine on the K-1, but it's not as good as the A version.
Regarding astro photography, I never tried to photograph the stars, only the moon with a huge Pentacon 500mm. Reading about the qualities of my Rokinon/Samyang 24mm for this specific use, I'm determined to try soon!