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.

then to BKK and back to Chiang Mai to return the bike but it means a bit more backtracking for flights/trains

Maybe better to rent the bike in Bangkok, then take it along to Chiang Mai (bus, train, plane), no backtracking needed that way.

Fairley ambitious loop via Luang Nam Tha, Luang Prabang, Pakse to Cambodia and back to Bangkok for a one month trip, but I'm sure you can do it if keeping to the main roads.

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.

but unfortunately, I can't afford a hotel in every city.

That could be very isolating anyway , better stay in hostel dorms more chance to meet other travellers and get info. In my experience theft from hostels is pretty rare . I have never had anything taken from hostels and guesthouses I have stayed in. But still keep your money and passport etc in a flat money belt under your clothes. If you have a valuable camera keep it in you day bag rather than leaving it in your accommodation. Better still wear it on a strap over your shoulder under your shirt when you are out and about. (cameras can get damaged in a day bag as they bump into other things in the bag)

Doesn't every country have lots of gyms?

Yes, but let me give you a few examples to clarify what I mean :)

In the states, there is many but the prices usually require you to haggle in order to get a decent price, and only some of them do day or week passes. On top of that, there is paperwork and they want to charge your bank (not to mention, its not walkable distance, but thats unique to here im sure)

Maybe its just me... but in Europe I've had issues in some cities finding a good budget friendly gym with equipment.

I was told in Chiang Mai you can get a good one for 80 baht a day. Is that the case in like... Krabi? Do hotels there tend to allow you to buy day passes to their facilities? Any paperwork / phone requirements / internet requirements or price haggling needed? I hope that makes sense :)

“When we left Chiang Mai the smoke was bad but tolerable. In Vientiane it was worse and Luang Prabang was worse still. Now we're in Nong Kiauw and the air is nearly unbreathable, with ashes falling all around us, and terrible visibility”

Raining ash, remember it well, gets me all nostalgic.

This is the hot season in mainland SEasia and has always been so. Is it getting worse? Is this year the worst ever? Could be, no denying it, but it has always been.

If a retiree decided to move here, and was bothered by it. I would assume they did not do very good research on their perspective new home. The Rose of the North might be nothing more than a PR line, but for nine months of the year Chiang Mai’s climate is great. All you have to do is pencil in on your calendars, Feb15 – May15 as the bad weather period and plan accordingly.

The hot season, is “Tiow season”; time to get out and go traveling. If you want it cheap, easy and close by; Air Asia has those BKK – Denpasar tickets. Indo has reverse seasons form mainland South East Asia, and you can find even more climatic variation in Nusa tengara, Sulawesi or the Maluku islands. And you can always go farther afield than that.

Just discovering this TWoV thing. From what I read, you cannot exit the transit area of the airport. If so, does it work with departure from Kunming? Last time I checked (two weeks ago), you had to exit the international area, pick up your luggage, check-in outside, and go through immigration to board your international flight... The route was Chiang Mai (Thailand) -> Kunming -> Paris (France) with only about 5 hours in China. My friend had to get the Visa free entry instead, which has restrictions on where you can travel to...