Since your first itinerary draft, you've added 3 stops (Bangkok, Chiang Mai, and Chiang Rai) that not only undermine your initial goals (few or no other tourists, authenticity, a contemplative traveler’s pace) but also eat up time that would be much better spent elsewhere.
Tuesday 17 - Wed 18 Chiang Mai (2 nights planned)
The detour to Chiang Mai for just 2 nights is completely pointless. Skip it by taking a bus straight from Bangkok to Chiang Rai.
Chiang Rai (1 night planned) – is that enough?
No.
If you plan to visit the city and (especially) its surroundings, you’d need at least 3 nights. On the other hand, this is where you could rent a scooter for Mae Salong and the area for a few days before returning it when you pass back through Chiang Rai, then heading to the Terminal 2 bus station there to catch the minivan to Phayao. Ideally, and still following your original plan, Chiang Rai would just be a transit stop for you.
Nan (...) to spend 3 nights there?..)
3 nights, yes—but 4 would be better since it’s a really cool city with beautiful temples that are different from those elsewhere in the country, plus bike paths. But it could also be your second good opportunity to rent a scooter for several days to explore its lovely mountainous province, so you’d need even more time...
Nan (I’m thinking while writing that Phayao is too close to Nan to spend 3 nights there?..)
The two cities have nothing in common—they’re not visited for the same reasons. Plus, they’re not *that* close, with a 5-hour bus ride between them. That said, if you’re short on time, spending just 2 nights in Phayao is doable.
Nan
(...)
Sat 28 - Mon 30: Loei (2 nights planned, not the Monday night)
You won’t be able to make the Nan-Loei trip in one day. Reread my first itinerary suggestion—I included a transit stop you can’t skip, and doing it this way will let you spend just enough time in Loei to switch buses.
Mon 30 - Wed 2: Pak Chom (2 nights)
Wed 2 - Fri 4: Nong Khai (2 nights)
Making such a long detour from the north just to spend a single, measly 2-night stop along Route 211 by the Mekong is pointless. Either you stretch it out to 8 nights total, as I originally suggested, or you skip the northeast entirely.
Sat 5 - Fri 11: Koh Samet (probably returning to Bangkok Friday evening)
6 nights on Koh Samet is way too much, especially since you’ll be short on time elsewhere.
Do you think I need to book any transport in advance for these routes?
Yes, the Phayao-Nan bus—but there’s no need to do it now. Just handle it directly at the Phayao bus station when you arrive from Chiang Rai. For the rest of the road transport, either it won’t be necessary or it won’t be bookable (regular buses or songthaews). In the latter case, you just hop on and find a seat (never next to a monk) or, rarely, stand.
Nong Khai—you mentioned taking the overnight train. Should I book my seat on that train back to Bangkok now?
Yes, because it’s already pretty full. Otherwise, take the bus.
Chiang Rai Bus Terminal 2—the minivan that goes (among other places) to Phayao:
Nan:
Nong Khai Station:
