@Florian_Etien and @mathieubossaert, do you two know each other in real life?!
Bienvenue, @Florian_Etien, nous sommes plusieurs francophones ici et nous acceptons volontiers un peu de français au besoin!
I haven't had a chance to think deeply about this yet but wanted to give a hint in case it might be able to get you going.
You can build a truly dynamic select from a repeat: https://docs.getodk.org/form-logic/#generating-select-ones-from-repeats You cannot use form references for the geometry column on the choices tab, only for the label column.
If the repeat has a question literally named geometry inside of it, then you can use the map appearance for the corresponding select and the repeat instances will be mapped. As @mathieubossaert has noted, though, there is currently no way to select multiple geometries from a map.
It's not currently possible to show already-captured geometry as context for capturing new geometry but we are starting work on that functionality very soon. You can learn more and share your thoughts at Display existing geometry when capturing new point/trace/shape on map - #15 by Aly_Blenkin