You’ve come to the right place! We all love name puzzles.
I really think Frances nn Franny/Francy would be perfect with Marigold & Jane. I also really like Louisa (instead of Louise) with Marigold & Jane. These would be my fave of the sorts of names you’ve mentioned. I don’t think Lucy or Lucie quite holds up to Marigold. It works with Jane but together are overpowered/ outbalanced by Marigold.
Also, rather than having to have a two syllable name, my sense is you could go with three syllables as well if that name has fewer than 8 letters, and visually still looks short and upright (eg Emily) compared to Marigold.
So, how about:
Clara - probably my fave suggestion
Alice - don’t like the name but it works
Lydia
Verity
Eloise
Eliza
Elinor
Elspeth/ Elsbeth
Laurel
Amybeth - this is gorgeous with your other two
Estelle
Rosalie
Beatrice
Letitia
Charlotte
Lucette
Amelia
Tessa
Harriet
Emily
Phoebe
Celeste
Willa - my daughter’s name (and first middle is Fr@nces)
With Louisa and Frances (and subject to preferred meanings) these’d all work.
Hope this helps! 