[Should I trade the 16-80mm for the 56mm 1.2]

What kind of portraits do you take? Is WR necessary in your environments? Do you shoot a lot in low light- why do you need a fast lens? Back in the day my first DSLR was a Nikon d70 with the 18-70 lens for weddings and studio portraits, it served the clients very well, and took some of my best wedding shots with it. I still use that same lens when I *have* to shoot dslr. If you're happy with the look of 16-80, which is a very nice versatile lens, stick with it for awhile and gather data on what focal length you most use for portraits. All the lenses mentioned here are good it's really down to the ideal selection for your purpose.

[Thoughts about Darktable]

I was a Photoshop user for over 20 years and then LR. I no longer do commercial work and retired to photography for fun now. I quit that suite earlier this year and learned Darktable. There was no point in paying for a subscription when I do less post now. I find the catalogue can be a shortcoming if for commercial. But other than that I'm happy with editing and does pretty much everything LR does, with more granular controls. I absolutely love the filmic RGB module. I tried Rawtherapee but it wasn't for me. There is a learning curve for anything new. For me and my purpose it was totally worth it. All the post processing apps do similar things just differences in features, gui and terminology. There are youtube videos and my favorite is Rico Richardson.

[Best Portrait Lens, Recommendation.]

My copy is smooth like other native lenses. Shouldn’t be too tight like you have to fight it. 

[XF 18-55 mm Motion Blur]

With auto iso set the max iso higher to 12800 as the X-T3 can handle high iso well. Perhaps open the aperture at the widest and focus on the cat's eyes. 

We all have scenarios, to shoot stills @ 1/20 with ~35mm. I would drop my shoulders, go gentle on the shutter release button at the top or bottom of my breath, and keep elbows closer to my body. If I can brace myself against a wall then it helps greatly. Shooting at low speed takes practice no matter what camera or lens. Once you understand the basic relationship between shutter speed, aperture and iso, with good camera holding technique you'd be so far ahead. However I don't shoot below 1/125 with my cat even if she is "still". At 1/60 I'm really pushing my luck, even if she is sleeping. BTW you have such cute cat!

[Best Portrait Lens, Recommendation.]

I highly recommend the Viltrox 56mm based on your list. I just got it and very pleased with people portraits. I think it's worth more than I paid for. You need to control for correct exposure and scene as it can be more prone to fringing more than fujifilm lens. I have the Fujifilm 50mm f2 which is very sharp with fast AF, but use it mostly for pet portraits. The 35 1.4 is wonderful too for wider portraits and general, shop around for used if $ is a factor. Good luck!