I think that there are already dedicated and matured programs around that synchronize any amount of files - and that at the click on a button.
I doubt that visual comparing the tags in large libraries is a feasible way.
To get the tag data from the source files to the target files, use Edit>Copy tag and Edit>Paste tag.
If the source files and target files are the same in number and order, you can copy all the tag data from all the source files to all the target files in one go, no need to do it file by file.
If you want to encode files (again) I also think that there are a lot of ready-made tools out there that already have numerous functions to cater for almost all the requirements.
Here is a thread about the use of ffmpeg as a tool to be called from MP3tag.