Thanks - will look into Nan...
Am now thinking it might be better to skip Chiang Mai. Kanchanaburi may be a better option and less travel.

I just left Luang Prabang on Sunday. The areas more north of LP are actually more polluted from the slash and burn. The town itself is actually still quite nice. Also, the Chinese New Year vacation time has wound down and there are not that many Chinese tour busses around atm.

I have followed the route between Phnom Penh and Pakse several times in recent years and its great. I highly recommend it. If you were to include Luang Prabang, it makes the rest of the trip a little bit rushed,, but ultimately it is up to you.

If you leave out Luang Prabang, you could also spend 2 nights in the 4000 Islands area.

If you make Pakse the "end point", you can either fly back to Bangkok with Lao Airlines, or you can cross the border to Ubon Ratchathani, Thailand, from where there are many flights and an overnight train to Bangkok.
There is an "international" bus that runs between Pakse and Ubon Ratchathani which costs around $8 and takes 3hrs. It waits at the border for the passengers to complete their immigration.

In each of the towns along the route I like hiring a motor scooter and exploring the surrounding area, but you can also have a good experience by cycling or travelling with a tuk tuk driver.

A few notes on each town:

Kompong Cham - Excellent day trip to Wat Hanchey 20km north, a large hilltop pagoda complex, with great view over the Mekong. Many interesting villages along the road there.
Excellent day trip to the island (Koh Paen) south-east of the city. Several large impressive pagodas and many traditional villages.
Another day trip across the big bridge and travelling south through villages along the east bank of the Mekong.

Kratie - Located in a very scenic location right on the Mekong.
Enjoyable cycling or riding a scooter along old riverside road north of the town.
A day trip by ferry to the island of Koh Trong, the large island in the Mekong opposite the town
Boat or kayak trip to view dolphins!

Stung Treng - A place that many people rush through, but is worth at least a nights stay. A town with in a very scenic riverside location and a large interesting regional market.

Don Kong - the largest island in the 4000 Islands group, yet does not receive many tourists. I really enjoy riding a motor scooter around the perimeter (~50km) through many small villages and countryside.

Pakse - I really like this low-key small city which sits in a very scenic riverside location. A place that does not have a lot of obvious "sights", but the ambience grows on you.
A nice half day trip to Wat Phu Salao, a small pagoda and thousands of golden buddha's waving to you (!)
Don Kho - a lovely small island in the Mekong, adjacent to the village of Ban Pho, 15km north of Pakse.

Lop Buri is a pleasant city and it was just an alternative suggestion together with the idea of the sleeper to Chiang Mai. OP can I am sure decide what option they prefer

We were in Luang Prabang last week. We didn't smell or affected by agriculture burning at all. Just morning haze and clear up blue sky all day. Be prepare for many cars and tour vans, mostly Chinese tourists and their Chinese registered vehicle that clog small streets.

What about this, this will be a lot a lot of flights. Are there other options?

Hongkong -> Bangkok (flight)
Bangkok -> Yangon (flight)
Yangon -> Bangkok (flight)
Chiang Mai -> Krabi (flight)
Krabi -> Bangkok (flight)
Bangkok -> Malysia (flight)

Note that the date has been put back and the main event is now on December 12th.

In the meantime, there is an exhibition hall on Sanam Luang (the big field North of the Grand Palace) with lots of info about the coronation and large scale-models of some of the boats. There is a free 20 minute musical/dance show every two hours (all day at the weekends (10 12 14 16) and three times in the evening 4,6,8pm). Get there early for a decent seat. It is all in Thai, but has great costumes and dance from around the country. There are also performances of various folk-tradition shows music outside (south of the exhibition tent) from about 6-11pm everyday.

airport-style security applies (take your passport, leave lighters at home).

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.