That is the the command I ran earlier, only I had all the input files listed together. Doing it your way, I successfully converted all of the manually to HTML, although two generated errors:

E[line ###] section out of sequence: expected level 3, got level 4

I don’t understand what the issue was, but the HTML generated anyway, so I moved on.

After successfully merging everything to a single HTML file, I dropped it into Calibre and converted to ePub—it appears to have worked!

If anyone wants the file rather than recreate my path, send me a DM.

Thank you!!!