I quite agree (tho have no background to say much) that you should be allowed access overnight only thing that came to my mind:
if she’s nursing still at 6months then over nights could cause problems with feeding schedules. BUT at 6 months your daughter could be having cow’s milk now so it needn’t be a problem, and your ex could express while you have the child so that you have milk -1st week it might not be able to work, but second week it certainly could/would.
If the nursing is not the trouble/hold up then I’m not sure where the trouble should be… and so agree mediation and compromise and then solicitors would be the way to go.
just saw you said she’s not BF- defo shouldn’t be a problem really then