As far as I know you have to go with saving as mxml (and I think you lose some of the musescore formatting with that).
I believe the conversion through ABCImpEx goes through mxml as well.
Anyway, the best converters I found have been the ones from Wim Van Vree (maybe these are even the ones underlying ABCImpE).
Have a look at https://wim.vree.org/svgParse/index.html
You can find command-line converters there (also a link to a GUI).
And you can find and try some online converters. The ABC tools by Michael Eskin are excellent und you can directly drop a mxml file into the the window and it automatically converts to ABC. I don't know if it works with the brackets you mentioned, but you easily try out yourself.
https://michaeleskin.com/abctools/abctools.html
I believe the conversion through ABCImpEx goes through mxml as well.
Anyway, the best converters I found have been the ones from Wim Van Vree (maybe these are even the ones underlying ABCImpE).
Have a look at https://wim.vree.org/svgParse/index.html
You can find command-line converters there (also a link to a GUI).
And you can find and try some online converters. The ABC tools by Michael Eskin are excellent und you can directly drop a mxml file into the the window and it automatically converts to ABC. I don't know if it works with the brackets you mentioned, but you easily try out yourself.
https://michaeleskin.com/abctools/abctools.html