Yes totally agree with Paps concerning the role of the charts here and in general. The idea is to have an efficient reading while playing, so the less symbols the better.
Willb I would recomend to start from an empty grille
https://guitarejazzmanouche.com/grilles/add/ and to save it regularly while editing.
To split a bar, first select an empty bar, then split it (Diviser) and after that you can start to put the chords in. Sometimes, when the chords are too complex, the last symbol can disapear but it's quite rare and very hard to prevent, so I would recomend to simplify the chord.
Good luck willb, this tool is not very user friendly as you noticed but I made it very fast, maybe too fast.