How to most easily save and re-apply a batch of tags from a data file?

I re-generate my song mixes WAV files over and over. Which means converting them into MP3s again and again. Each time I must go back into MP3Tag and manually re-enter ALL the tags. This is getting very tedious and painful.

What I really need is this: after I've manually adjusted,edited, etc some tags for a bunch of MP3s, I'd like to SAVE all those tags into a file. Then later, when all the MP3s have been erased and recreated, I'd like to reload all those tags from the saved file, and viola, there are all my tags once again, ready to reapply to these newly generated MP3s.

Export to CSV does the first half of this, but how to accomplish the 2nd part?

I've tried to use the Text <-->Tags features but find them very confusing. I really just needs a safer simpler way to save all the tags, then reload them later. How best to do this?

If you keep the old mp3 files with all their tags then you can simply copy the tags and paste them to the new mp3s.
You can do more than one file at a time if the order and quantity of the source files matches that of the target files.

CSV is probably not the best format as it relies on the comma as separator between the fields which can also be part of the data in fields. Any additonal comma will then confuse the import. You would have to find a field separator that is printable but is not part of the field data.
To align the data in the text file with the audio files, you have to include the filename as identifier.
Going via export / import means that you have to know exactly which fields are present in the files. If there are fields in a file and you forget it in the export script then that data is lost.
The cover is usually not imported, this has to be done separately.

I would say that the easiest way is copy&paste.

Copying and pasting would be just as tedious as what I do now.

I am not sure if you got that right:
If you have, let's say 500 files and you have them in a folder called "Old".
And you have another 500 files with the same name and in the same order but in the folder "New", then you
Select all files in folder "old"
Copy the tags from all files in one go
Select all files in folder "new"
Paste the tags to all files in one go.
I see nothing tedious about that.

Are you saying the entire body of data can be copied and pasted in one step?

Not quite - I described the steps in the previous post. I also described the conditions (most important: source and target files have to be in the same order as the data copied from the first file is then pasted to first of the target files, the data from the second source to the second of the target files and so on).
But yes: you could do it for several hundred files in one flow of select sources-copy-select targets-paste
In other words: you do not have to do it file by file and tag field by tag field but you can copy a whole tag and paste that whole tag to a target file
And that even works for a whole bunch of files in one go.
And the good thing: it also copies the embedded picture.