Saving tag changes at once

There is a slight problem when saving tags with multiple changes at once, i.e. if I change/add album title, artist, cover art etc. and renumber, then everything else except renumbering, is lost. It seems that first I must do the 'one time' tasks (cover art, album title, artist name etc.), save the tag, and then perform the 'multiple' tasks (renumbering, track titling etc.) & save again. Can this be improved?

Entering the data in the tag panel is a separate step from other editing steps.
I doubt that you can change the TITLE for several files unless they have the same title.
So I see very little room for "improvement" here.

If you have entered data via the "Tag Panel" user dialog, then make sure to save the current changes into the selected files, before you apply other dialogs of the Mp3tag user interface.

DD.20151208.1714.CET

OK, Let's give an example. I have to write tags for an album i.e. to insert/modify album art, album title, album artist and import track titles from a text file. I believe the whole thing must be considered as a 'project' and be saved - after finishing the imports/modifications - in one key stroke. As it is now, it is a sequential process: first the album art, title & artist - save - then insert titles - save again.

MP3tag does not know anything about albums. It treats each file individually.
You only have a GUI that allows you to enter data for common fields in one go - this is the tag panel or the extended tags dialogue.
You can even assign the same data (e.g. the year) to numerous tracks from different artists and albums.

Using the GUI, there is no way to select several files and assign different data to different files. You have to use the assistant or actions which run without GUI interaction.

You can create actions that read data from a text file and increment a counter to emulate the track number assistant. But I do not see that as an improvement or shortcut.

So it is up to you to split up your workflow in such a way that you can minimize the amount of data you have to input. Even if you see it as a "project" it is only a chain of user actions for MP3tag - just as you described very accurately. That is why MP3tag is so flexible.

BTW: the section "Development" is not intended for feature suggestions. See "General Discussion" for that.