[BRX]

Well, the mentioned ABC Tools by Michael Erskine are the simplest für single quick conversions.

https://michaeleskin.com/abctools/abctools.html

Mandolintab.net does too

https://www.mandolintab.net/abcconverter.php

I'm not sure what scripts they are using, but it guess it's all the scripts by Wim DeVree

https://wim.vree.org/svgParse/index.html

which I have used (and which are continually updated and improved. He provides command line tools as well for batch conversion.

Have a look also at

https://blechtrottel.net/abc2xmlgui.html

and

https://blechtrottel.net/abc2xmlgui.html

The former is a plugin for MuseScore (so they are using Wim's scripts as well), the later a GUI for the command line tools.

Of course conversion from notation program to mxml and to ABC can't be perfect because of the different approaches (graphical notation against text based one) but you can get some pretty good results.