The main reason I went with the square filter system is I have (or at least had) a penchant for changing lenses constantly (plus I was working across both FF Canon and Fuji systems for a period so that involved different lens sizes as well) so it was a PITA to keep investing in different circular filters for each lens size. So now I have whatever size adapter rings I need and just one set of square filters which is much easier to work with - when I say a system I only have three plus a circular polariser - ND4 Graduated filter and ND6+ND10 full filter.
Your synopsis of the Samyang pretty much mirrors mine. I used mine for astro which, as you say, was excellent however for landscapes it was just average - unless you had some great light to work with. I found colours and contrast were alright but nothing special. It's not the sharpest lens in the world but nice and sharp for the price.
I sold it and bought the 10-24 f4 which I have enjoyed using. It's sharper than the Samyang at 12mm plus goes that little bit wider which I like. Beyond 18/20mm it does get quite soft but I have the 16-80 f4 to take over from that point.
For my astro I have been experimenting with my 23 f1.4, which is much better, but when I need wider I've been stitching shots together which has been fun.
As I said above I think the 16mm would be a terrific investment for anyone who would get the use out of it. I'd love one but it would leave me with three lenses that do 16mm and there's no way I could sell the 10-24 because I'd miss the 10mm for sure.
What sort of outlay would I be looking at to buy a filter system with say the three filters you have? I now have the 16mm f1.4, 56 f1.2, 23 f2 and 55-200 so assume I'd be able to buy a system and adaptors to accomodate all of these? Sorry I'm fairly clueless as have never used one before.
Lie's 16mm 1.4 turned up yesterday and took it out for a spin. I can see why everyone loves this lens, there's something about the images, even straight off the camera as JPG they look fantastic. Here's an image I shot wide open - it isn't sharp at all and I should be at something like f/5.6 - but if I take my technical hat off and just look at it as an image there's something really pleasing about it, the bokeh is beautiful. I'm not sure I'd be able to get anything like this with my other lenses.