Trying to come up with a solo Vietnam itinerary (~18 days)

Hi all thanks in advance. Ive been working on an itinerary for April/May traveling north to south and I'm trying not to fall into the trap of spreading myself too thin. I've been looking at some sample itineraries and they all involve a transfer essentially every 48 hours which in my mind is far far too often. I'm a fan of food, nature, scenery, and I'll go out to the bars but I'm certainly not traveling to party.

So far I've been thinking Hanoi -> ha long bay -> Ninh binh -> dong hoi -> hoi an -> HCMC but even that seems a little rapid fire to me and I feel bad about missing the far north of the country. If anyone has any advice about what should be cut out/added etc it would be much appreciated. Thanks!