I think your partner's plan is a very reasonable one because you are doing the drive in day light which is sensible and when you are fresh.
I would prepare a holiday Xmas stocking of little fun items all wrapped like little pass the parcel gifts. Open a new one every half hour. These could simply be things like removable stickers that would stick on her window or tiny little picture story books or age appropriate playdoh material. Don't spend very much and just recycle old magazines as the wrapping paper. Be prepared that the car might be a little bit untidy but the adult in the back can tidy up from time to time.
Also you could get one of her fave soft toys/ dolls and let her pack its suitcase/ toy cage and take it with you. Give her a role to play for the journey. She has to remember get the toy animal/ doll a drink, go to the toilet, keep it warm, help it get off to sleep when you stop the car etc. Maybe it's got a temperature or is homesick and needs a cuddle. Encourage her to think about how it feels and deflect her from thinking she is in the car...
Plan to rotate the person in the back seat every few hours.
I'd try not to make a big deal of the journey and be very matter of fact especially with your conversations with your toddler. It gets both harder and easier the more you do. Harder when you introduce another brother or sister into the mix... Also having another adult potentially judging your parenting skills adds a frisson of tension.
Might also be worthwhile reading all the 'babies on an aircraft' threads as they are similar scenarios...
Also worth remembering you might need another set of parcels for return trip...
Have a great trip and remember to pack your own things and not leave your own needs to the very last...