I'm also not understanding what advantage any direct DB integration would buy, and think such a feature would unnecessarily add to MP3Tag's complexity.
one very useful option: I can choose from my existing library to fill up most of the required fields (considering I have the same artist, composer etc. in some of my previous files)
If you just add multiple files/directories to the MP3Tag UI, a mix of already-tagged and new files, you can then easily get the data you want from the old to the new via the sidebar and/or the Extended Tags dialog, or in some cases using Actions.
I've found MP3Tag's UI features to actually be better than most database tools - maybe you just need to get to know it a bit more thoroughly.
Or just invest a bit of time up-front in setting up a well-automated standard import/export workflow - then you can do your data-entry in whatever tool you like and still use MP3Tag to actually write the tag data.
IOW you should be able to do exactly what you want with MP3Tag's existing feature set.
If you still don't think so, perhaps give some detailed step-by-step examples of what you're looking to accomplish to help us understand your perceived unmet needs.