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live view doesn't have enough resolution to accurately tell if in good focus especially with a 50mm lens.
infinity markers on lenses are NOT the infinity point, just an indicator of which direction you need to turn the focus ring. The end stop on lenses is actually beyond the infinity focus point. this gives you the chance to tweak back and forth and see for yourself where the ideal focus is. Live view was made for dummies and this is a camera dont forget, its design is meant for an eye to the viewfinder to see through the lens TTL. They are not designed with live view in mind as primary view, just because its there don't assume it is.
Also Live view is Never Live, the sensor has to capture what is sees, the processor has to process and then the screen has to display it. All these parts take time to do their thing so as you adjust focus it can take maybe half a second for the screen to catch up and display the change. But most people dont understand any of this and so dont pause when adjusting just keep moving the focus instead.
the donut is the artifact that shows you went beyond the focus point of the lens. You should only ever use live view for composition as its useless for fine focus. You can use it to display and zoom in on a test photo as this is more accurate, live view is a preview not a captured photo. do a series of fine adjustments and view close each test to find the sweet spot. Then maybe use a sharpie to mark on your lens where the spot really is.
Once you do this keep that as a reference sample and set camera to spot focus and pick any bright star to spot focus on, easy to do using any lens. and take test shots again, some cameras have back/front focus adjusters in their menu to fine tune a lens' focus on that body. some also let you configure camera buttons . for example my nikon d800e I have tuned and tested and configured it to have a convenience constant focus button for me and remove focus from the shutter button completely. I know my autofocus does as good a job as my eye did manually focusing which I cant do after my stroke. So I just point to a bright star and hold the button down a few seconds for it to setlle and get good "infinite focus" then I can just point to where I want, use live view to composite and use remote to take the shots.
Donut images are not entirely useless, you can still process as normal and then halve the image size at the end to close the stars down to dots again instead of donuts for a usable shot. and it'll still be a resolution you probably would've killed for a few years ago.
A uv filter is just another layer of glass, so you'll lose photons and it does zero for focus. you should check your lens attaches to the camera firmly. there should be zero hint of a wobble there but if you've bent the connectors even a fraction of a millimetre your focusing will be screwed because the lens to sensor (focal plane) distance is now altered.
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