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. I suggest saving each edit you make to any section as a new BIAB .sgu file so you can easily return to an earlier version of a section if necessary. BIAB .sgu files are very small your song project is dependent on a well organized file saving structure. I suggest a folder for each section saved under a Master folder. This way, if you make 7 edits to Verse 3, you will have 7 BIAB .sgu files n the Verse 3 Folder and 7 audio files with the same name as the accompanying BIAB .sgu file. In your work flow, you would do your edits in MuseScore, copy the entire section that you're editing in (intro, verse, chorus, bridge), render the audio and place that audio appropriately in it's place in a Master Audio File. The Folder structure of your song will read as an Outline of your song and you are free to work on any section, in any order very quickly and efficiently. This also differentiates between BIAB generating audio and the saved audio you will use to linearly construct your song in a DAW which will also separate how you will edit that audio.
For instance, using your song structure of intro, three 12-line verses, a 5-line chorus that is repeated three times, a short musical interlude after each chorus, and a 2-line tag at the end. All of these sections combined total 327 bars. The total number bars is irrelevant because the BIAB .sgu file for any section is comprised only of the number of bars of the particular section. If your INTRO is 8 bars, the accompanying BIAB .sgu file and any rendered audio for the INTRO will always be 8 bars. The INTRO is bars 1-8 in the Master Audio File so any subsequent edits you make to the INTRO, the rendered audio of the edit will overwrite the existing audio located in the INTRO. Each section will work in the same manner. Verse One will consist of 12 lines as will Verses Two and Three and so on.
What you're doing working in sections mimics somewhat how BIAB generates a style over a chord chart. The BIAB search engine reads the chord chart and searches hours of audio to find appropriate recorded audio (RealTracks) of that channels assigned instrument that matches the chord chart, key and tempo of your song. Rather than have BIAB search RealTrack audio, you're using BIAB section exported audio.
Another recording technique I suggest we discuss is not a widely accepted or used concept of BIAB. It's been mentioned earlier in this thread or the other you posted that BIAB is limited to seven channels and a single audio channel for a limit of 8 total channels to record BIAB generated tracks. That limit is non existent. BIAB can function as a multi track recorder and applying multitrack techniques along with the feature that each channel can have up to 10 instruments (that can alternate or play simultaneously), it's easy to generate a BIAB chord chart and render audio comprised of 70 instruments on 30 tracks. This can be done in minutes not hours and provides very complex arrangements. Using .XML should be quite useful if you are using this technique.
Once we've discussed these, then we can expand to how RealBand can take these audio and .sgu files to even higher and more complex arrangements.
BIAB Ultra Pak+ 2021:RB 2021, Latest builds: Dell Optiplex 7040 Desktop; Windows-10-64 bit, Intel Core i7-6700 3.4GHz CPU and 16 GB Ram Memory.