I expect after numerous people turn up at customs with only the e mail in the future they will attach the visa to the
e mail. I think the previous 30 day visas were ? Had i designed the visa i would have made it the size of a passport page that way it could be attached to a passport page, maybe with tape. I am happy that i do not have to deal with visas for 5 years, unless of course they change the rules again and i am not holding my breath ;)
Lijiang is no longer what it was like in 2008 - these days the tourist activity is overwhelming, the old city is anything but preserved (it's mostly been destroyed so that new, faux-old houses can be built for tourist businesses), there are plenty of cafes and bars, though hardly laid-back, and the place is famous in China as a destination for those looking for one-night stands.
Assuming you're looking for something like the old Lijiang, rather than its current form.... Northern Thailand and Laos might be to your liking. Pai is a small town, very touristy, but still pleasant. There are less touristy options nearby (Mae Hong Son, for example). Chiang Rai is another nice area: Chiang Rai itself is a city, albeit a small one, while nearby towns like Chiang Khong and Mae Sai are very relaxed. Northern Laos has places like Luang Namtha and Luang Prabang, while there's the 4000 Islands in the south (more tourists, fewer locals, lots of music). Just a few ideas, there are many other options.
Personally I am not a fan of Ayuthaya and I agree save BKK until the end
If you are going to Chiang Mai I would suggest you go to Ayuthaya in the morning and look around and catch the sleeper that night but instead of Ayuthaya I would go another hour up the line to Lop Buri (or in the opposite direction stop off and continue the journey to BKK
Somewhere in the Bangkok Post article it says 'at some points' the world's most polluted.
And that's exactly what it is, a snapshot of a moment of the worst month in Chiang Mai, during the burning city.
Here's something a little more interesting, from Greenpeace:
https://greenpeace.or.th/s/right-to-clean-air/PM2.5CityRankingsREV.pdf
It shows yearly average of pm2.5 and worst month average of pm2.5 for 2014-2016.
The yearly average is 30-55, while the worst month average is between 144-266 depending on year.
While that's certainly nothing to be happy about, and the worst monthly can be sky-high indeed, there are cities in India that have a YEARLY average of about 140-170.
But indeed the burning season up North is truly awful.

In your short time frame i think it is quite good yes it is probably a bit rushed but that is as much to do with how much time you have. The only thing i would change is i would probably skip Bangkok at the start and take a mid-afternoon flight Chiang Mai and then add that time on in Bangkok at the end. For Koh Lanta you can pick up a taxi at the airport or you can book one through 12goasia for £15 for the both of you. I really liked Koh Lanta, quite a bit to see on the island and some good day tours. Only Island i have been too where hiring a scooter felt pretty safe.

7 months after first being "opened" in a mock ceremony on March 19, the 2nd Friendship Bridge between Mae Sot and Myawaddy will finally open to traffic on October 30.
https://www.bangkokpost.com/business/1771159/second-thai-myanmar-friendship-bridge-to-open-oct-30
What this will mean for tourists is unclear at present, since the article strangely does not mention tourism but only trade, although when I visited the bridge back in April, I was told by the guards that 1) all trucks will be required to use the new bridge and 2) tourists, business people from Thailand, Myanmar and third countries holding passports and visas will be required to use the new bridge, 3)while day visitors from Thailand and third countries to Myanmar and day visitors from Myanmar to Mae Sot using border passes will use the old bridge. 4) Cars will be required to use only the old bridge, meaning they will be restricted from using the new bridge despite it being so big and grand and far better equipped to handle large traffic volumes compared to the old one. Cars may be permitted to transport people across the new bridge, but only as far as the immigration and customs complexes of the other country.
As far as I'm aware, vehicles (other than local ones traveling on a day return basis within a limited radius from the border) will continue to be restricted in the same way as now so unfortunately the new bridge opening won't change these regulations, for now. The only thing that has been mentioned is that trucks from Thailand and Myanmar may be allowed to travel to each other's ports, which, if implemented would be a first because at present foreign trucks are prohibited, either by regulations or in practice, from transporting goods all the way to Thailand's ports. Therefore, cars and motorcycles wishing to tour Myanmar will continue to require permits as now, although they will likely be required to cross at the new bridge after it opens.
Expected opening hours will be 5.30am to midnight Thai time or 5am to 11.30pm Myanmar time. This means the bridge will have longer opening hours than any of the other friendship bridges linking Thailand and it's neighbors, although it has been reported that Lao and Malaysian border crossing points will finally open 24h at least on weekends and during holidays, a long overdue move. With increased trade volume at this bridge, I expect that over time, the bridge opening hours will also become 24 h.
Once I know more, I will update this page. My next trip to the area will be before the new bridge opens, while the next one after that probably in a couple of months.
I also did the trip 30 years ago. It was very cheap and I quite enjoyed it. Current price are obviously a lot more.
According to Travel Fish the public boat from Thaton to Chiang Rai costs 400 baht/person (departs 12:30 pm) and private hire of whole boat 2400 baht, takes around 2.5 or more depending on river levels. You don't say when your are going which would determine the water levels. Since you want to go the very next day, I would head to the pier as soon as you get to Thaton and see what you can arrange. Or, if you already have your accommodation in Thaton, you could call them and ask about arranging the trip. You could even try asking at a travel agent in Chiang Mai. The last two options will most likely cost a bit more, but it would give you peace of mind.
I don't know about private car hire from Chiang Mai. I would guess that it won't be cheap. Quotes I saw on line hovered around 2900 baht. I couldn't get a quote from Uber and don't have the Grab app.

Hi, traveling through Vietnam March/April 2019, did three weeks in Thailand in 2018. Due to some complications we didn’t travel north of Bangkok towards Chang Mai and regretted getting to see that side of the culture.
So...We haven’t fully decided to start south and go north or vice versa but I would love to see some of the north of Vietnam around Hanoi whilst we are there.
Has anybody any suggestion of where to visit and the logistics of this? Is flying into Hanoi the best option or is Hong Kong worth thinking about? From the Uk. Thanks :)
Chiang Mai is a cute town with a lot of western influence, you can visit the mountains and do some trekking or visit elephant nature park, Thailand's ethical elephant sanctuary. It's very expat influenced, very cheap and has great food.
Chiang Rai is pretty much just the White Temple and otherwise very quiet. (3hrs away from Chiang Mai so you could do this in a day)
I'd say Luang Prabang is the most unique and would do this if I were you. It's a UNESCO heritage town and also you can visit the most amazing waterfalls in SEA.
Check out my blog for some tips on what to do there - http://myaltruistictravels.com/2016/02/24-hours-in-luang-prabang/