Planning an Authentic One-Month Trip to Thailand and Vietnam: Seeking Recommendations and Itinerary Ideas

Hello everyone,
I am planning a one month trip to Thailand and Vietman this summer, starting July 18th. I arrive and leave from Bangkok and I would like to go to Vietnam afterwards where I plan to arrive in Hanoi, before discovering Halong Bay (from Cat Island), then the mountains of North Vietnam (Son La , Dien Bien Phu, Lai Chau, Sapa)…

I am looking for authenticity. I’m not a fan of too touristy places, apart from Halong Bay that I would still like to discover. I love diving, hiking and also looking for beautiful beaches in Thailand, not too touristy.

What route do you recommend? Especially in Thailand, before taking an internal flight to Hanoi.

Thank you in advance for your help.