I use this cheat-sheet: https://sproutsocial.com/insights/social-media-image-sizes-guide/
Bottom line: The image will be displayed at 1080 pixels wide and up to 1350 pixels tall, depending on the aspect ratio. So that is what you'd input for resizing. Of course it should be JPEG sRGB; the quality slider you can experiment with, but I doubt you'll see a difference in practice on most photos between 70, 80, 90, and 100.
It might be zooming in/out with a zoom lens during the exposure.
In the modern mirrorless systems, a lens correction profile should be embedded in the raw file, so that any raw converter can apply it by default. Do you actually notice severe distortion or vignetting when using Photomator or Darkroom, that you didn't see in Lightroom? It might already apply lens corrections, without giving you an option to disable.
I mostly shoot landscapes, and basically never shoot indoors or with other artificial light, so my experience is probably not relevant to your specific question. But it might help in a broader sense.
As someone here pointed out, the "correct" white balance might be defined as one that makes a white object appear white. Suppose we all agree that snow is white... But what colour should it appear in a photo taken before sunrise? And how should it look right after sunrise, when it's directly lit by a very low sun? To me, that's some shade of blue/cyan and red/orange, respectively.
I also record and process raw files. The white balance setting on my camera is pretty much always on Daylight; that's the best starting point for me (again, in natural light outdoors). Any white-balance adjustment I make from there is usually subtle.
If you're absolutely certain you will buy an a7-series camera, you might as well get a compatible lens now. The Tamron 28-75mm f/2.8 G2 is an excellent option, as is the Sigma 24-70mm f/2.8.
But if you still haven't made up your mind entirely, then perhaps it's better to get a lens that will be more useful for you now. Buy one used, so that reselling it later isn't as painful. Consider the Tamron 17-70mm f/2.8 and Sigma 18-50mm f/2.8.
Unless you specifically want that high contrast in your photo, come back in better light. That could be an overcast day, or just another time of day with the sun in a different position.
https://www.reddit.com/r/photography/wiki/technical#wiki_how_do_i_shoot_in_low_light.3F