Not to worry. Your work in MuseScore and .XML files is a brilliant idea and allows you to construct a song in such a manner it's much like a roadmap of your completed song. Because of the length of your song projects and also the fact there's so much post editing, an .XML file is crucial to staying on track to how the song is constructed, the arrangement of how instruments weave in and out of your arrangement and how you dynamically build your song. Don't make any changes to how you presently use .XML files to input your chords, melody, key signature and tempo.
What I suggest is that rather than think of BIAB to generate a song, think of it to generate each section of your song. Rather than record all of your song as one linear song, record each section of your song, export that section's audio and in a DAW, stitching each section together. That's a recording technique called overdubbing and it's used extensively in studio recordings. Each section of your song will be comprised of it's own BIAB .sgu file. This replicates the concept of a live musician recording a verse separate from a chorus, separate from a bridge, separate from the intro and outro. This alone eliminates the 255 bar limitation of BIAB. Generating and saving the audio of each section of your song allows you to drop that audio into it's place in an master audio file. That bypasses the long entire song renders you currently have to do. If you make edits to verse 3, you only work within the BIAB .sgu file of the saved Verse 3. The rendered audio with the updated edits then overwrites and replaces the current audio populating Verse 3 of your master audio file.
As you know, Charlie, silvertones recommends that I drop BiaB completely and use RB instead. That may be beneficial in some workflow scenarios but perhaps not others. So on the one hand, before I decide to do that, I need to do enough testing with the procedure he is suggesting that I use on my songs with RB to make sure that's the right decision for me and my needs. On the other hand, I need to have a better understanding of the procedure you're suggesting I use so that I can go through the steps involved in that procedure in order to make a proper comparison between your suggested use of BiaB and his suggested use of RB. Therefore, I have some questions for you about your suggested procedure because I'm having trouble following all the steps in your explanation.
For example, in the above excerpt of your recent post, you mention a "master audio file." I assume you are referring to an audio file in a DAW such as RB or Tracktion. If so, then would each generated section of my song exist on its own track to make it easier to update? If so, that would eliminate the free version of Tracktion as a viable DAW because it only comes with 8 tracks, and I don't think more can be added. (This wouldn't prevent me from using RB, of course.) If not, then the best way for me to update an audio section in Traction would be to overwrite the contents of the existing audio section file that I had previously imported because it already is positioned at the proper location in the song. This would not be possible, of course, if I need to add or remove a measure during my editing of that audio section or of another audio section. In that case (which occurred a few times in the song I was working on a few months ago), other sections of the song would have to be moved in order to avoid gaps or overlaps in the song.
Another question I had pertains to my importation of an .XML file. You discussed the need for numerous folders to contain all the different sections of my song in BiaB. In order for me to create those different sections, I would need to create multiple .XML files within MuseScore for each section of my song and also have a complete master .XML file as the main road map of the song. I would also need to make changes to the master file and then copy those changes to one or more of the sub-sections as I go along, and then open the .XML exports from the edited sections into BiaB to regenerate the audio for them, etc. Am I correct?