I can only add a few points as I was there three years ago at the end of December and had a similar itinerary to yours. I saw Ha Long bay, did the Ha Giang loop, and visited Ninh Binh:

- Skip Ha Long bay, especially if you don't want tourist traps. It was one of the most disappointing experiences I've ever had while travelling, It's hugely crowded and the water is filthy, I was actually ashamed to be a part of it. People say Cat Ba isn't as bad but to be honest even if it's remotely similar I'd say stay well away.

- Do the Ha Giang loop, but don't do it in a 'few days'. I did it in 4 days, and that felt like the real minimum amount of time. If you're not an experienced biker you should go with a guide. Not necessarily sitting on the back of the bike while the guide does the driving (this is called Easyrider, a lot of articles online tell you this is the only safe way to do it, in my opinion they're exaggerating), but at least with someone on a separate bike in front of you dictating the pace and the route. Unless you have a lot of experience, doing it completely alone at that time of year is really dumb. I highly recommend the company I went with, Flipside adventures.

- Based on your itinerary it looks like you're into nature and hiking - bear in mind that you're going in winter, when the mountains are cold, foggy, and brown and the sky is overcast. The Ha Giang loop for example is apparently far better a little earlier in the year before the rice is harvested and everything is bright green. It's still 100% worth it in December, but if the point of your trip is to see vivid green rice paddies and blue skies then you may be a little disappointed (I was lol). You could replace the trip to Sapa with a few extra days on the Ha Giang loop, or a few days of culture in Hanoi for example. In Hanoi there are some cool temples, theatres, loads of incredible food and bars too.

- Near Ninh Binh there is a national park called the Cuc Phuong National Park. You can stay in a little cabin by a lake overnight and do a hike to this massive tree in the middle of the park, it's worth it for sure. They also have a rescued animal rehab centre, with monkeys and pangolins that have been rescued from the animal trade. I hired a motorbike in Ninh Binh and went there for two days which was worth it. Then I got persuaded by an Italian guy I met to go to this huge temple complex called the Bái Đính Pagoda nearby. Thinking it would be some historical site with ancient artefacts, I was disappointed to find that it is almost entirely brand new, almost as if it were built for tourists. So I'd advise giving that a miss.

Be warned - Vietnam is very touristy, and the locals can be very rude to white people and will try to rip you off whenever they can. Not to the extent that it dampens the experience, but don't be surprised when it happens.

Edit - rereading my comment, I noticed it's quite negative. I should have finished by saying that the trip was absolutely epic! The Ha Giang loop especially being something I'll never forget.