My operation is similar. I don't create songs so my songs use existing sheet music, but the process should be much the same.

I start of with a'song' of about 100 or so bars with no repeats. This is equivalent to an unfolded song, although it was never folded.

Then I add my melody, usually starting on bar 1, knowing I can add an intro later, unless I know what it is already. Copy and paste as needed, but save frequently

Once I'm finished,including all the passes and endings, it's time to play it through from beginning to the end. This is what BIAB will play regardless of how you define repeats, endings, DS al, etc.

Once it's to your liking SAVE (and SAVE again).

Then using the repeats / endings dialog you can develop the visual representation. It's purely a visual and won't affect the underlying song. Feel free to start again using the saved version - or update the save version if it's wrong. This process is often trial and error and many users give up. It's not always intuitive but it is repeatable.

I have note based lyrics for all my songs, so I typically use 3 different endings with the bridge being an extended 1st or 2nd ending.

It's not hard - but the key is a fully functional 'unfolded' base.

BIAB for Windows Version 2019 (643)
Windows 10 (1909)