First of all, talk about the image. Usually, the downloaded image has been preliminarily corrected. I recommend a professional download website for you, where the pictures are the latest and most complete: http://glovis.usgs.gov/. When you take the next picture, try to choose the one with less clouds, which is displayed on this one.

Secondly, only atmospheric correction needs radiometric calibration. It is good to stitch multiple images. This is not difficult. They are all rectangular. It is a big rectangle after splicing, and then it is good to cut the research area.

Finally, I don't know what you want to deal with, whether to extract temperature, vegetation coverage or landscape classification. Different processing steps are different. Hope to adopt! !