Gradually heading north instead of taking the overnight train?
Sukhothai could be an option, but since you’ll have already seen the ruins at Ayutthaya and seem to want to stay in the north, I think you can skip it.
In the north, do you recommend Doi Inthanon, Huai Nam Dang, and Doi Chiang Dao parks? Which of these parks is the most beautiful and recommended in January?
I haven’t been to any of the three. Marien33 visited Doi Inthanon earlier this year and seemed disappointed—if they don’t reply here, check their current travel journal.
We’d love a trek, but I’m not sure where or how to organize it. What’s certain is we don’t want a group trek.
You say "a trek" in Switzerland?
The bolded part might be an issue unless you’ve got a well-stocked wallet. Typically, "group" treks have 4 to 5-6 participants plus guides. Either way, to avoid jungle walks falsely sold as "treks" just because the term is trendy, don’t pick one that’s shorter than 3 days/2 nights. For the same reason, avoid any treks that include car rides*, elephant riding, bamboo rafting, or similar gimmicks.
*However, it’s normal for the trip from a city to the actual trek starting point—and back at the end—to be by songthaew.
I just read Biboun51’s travel journal, and they didn’t seem to enjoy Pai or the area. But opinions vary...
I hate Pai for multiple reasons.
Jojoone1:
Having recently done the Mae Hong Son loop, I don’t think it’s one of Thailand’s absolute must-sees.
It might be worth it if you dedicate at least 10-12 days (without Pai).
Does anyone know the Mae Sariang area?
I know the town—I’ve stopped there four times, for one to three nights. I’ve also traveled Route 105 (and some of the villages along it) from Mae Sariang to Mae Sot, once in each direction.
Do you recommend an excursion from Bangkok to Songkram for the Talad Rom Hub rail market?
Samut Songkhram, more commonly called Maeklong. The market’s fun, but you won’t be the only tourist there ... I did it by taking the train independently, which isn’t easy (two different local lines plus a short ferry crossing between them—it took me half a day just for the round trip, but I couldn’t imagine arriving at a rail market by road...).
Photos from the first train ride here: https://voyageforum.com/forum/loin-lotfai--lotfai-quotidien-d10777060/ I’ll post the second train ride and some market shots soon...
Do you recommend an excursion from Bangkok to (...) Amphawa for the floating market?
No.
It’s super touristy, and it hasn’t been a real market for a long time. Only opens on weekend afternoons. Book your accommodation (overpriced compared to elsewhere) at least two weeks in advance.
What I found way more interesting in Amphawa was the King Rama II Memorial Park... though almost no one goes there.

Jojoone1:
Pai isn’t really suited for those over 25
+1.