Had a three year old holland kindle, Sadly the kit died at 12 weeks. She never had a live kit again, thought she got pregnant every time but the last. When I culled her, and opened her up, you would not believe the amount of fat in that body. She had five tries, so good luck with yours.
I've never had a buck sire a litter before 6 mos, but I don't get testicles before then. If he's got his pieces, I'd let him joint the party.
I've never had a doe conceive before 5 1/2 months. I have some 4 1/2 month olds that the color is right, but they aren't the least bit interested. And one that was interested didn't take the first time. For the most part, I've always been told to breed my animals at 7-8 months for medium breeds. Put them on the show table as jrs and srs a couple of times, and unless I plan on not getting many babies from them, I'd better get them bred by 7-8 mos.
I have held the hollands, on every breeding except one. None of them have ever lifted. They do always get pregnant, despite what the information says about force breeding. The problem is the buck must be willing. One Rex buck I have is not aggressive enough to try to breed with my hand in the cage. Definitely not going to happen if I take them out. It seems that the act of moving the doe from the cage to place in the bucks cage makes her too testy to mate in some instances. Or she spends a lot of time looking around, which is what my youngsters do. I do not remove my rabbits from the cage frequently, just for routine check ups once a month.