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The Mae Hong Son loop!
I've done that - not as an audax but as a tour, over 4 days. I'd definitely recommend - wonderful route, lovely roads, some brutal climbs (the short ones, rather than the long ones), great food, lovely towns and villages, friendly people.
A bit of main road to get out of Chang Mai, then two great, long climbs in the first half ,then some really brutal short climbs round about 200km. One of them made my rear mech collapse out of shape.
Then it is some easy stuff through forest, before my favourite bit, which was after you turn left to cut back towards Chang Mai at 324 km. A really quiet, hidden valley, again with some steep climbs that had me on the edge. A very pretty section by a river.
I did it 10 years ago in December. I didn't have such good info on the route then so wasn't sure what to expect on some of the sections in terms of gradients and resupply options. And I didn't eat enough, so by day three I was struggling for energy. I'd done PBP that year so thought I knew how to ride long distances, but didn't really understand nutrition then.
I didn't do the out and back bit at 210mi / 338km, and I turned left at 282mi / 454km with the intention of going up Doi Inthanon (the highest mountain in Thailand). But that road turned out to be brutal - about 4km at an average of 13% - and I was on a steel tourer with panniers / luggage for a 4-week tour. Probably the hardest ride I've ever done. I quickly ran out of food, there were no resupply options. I ate a couple of tomatoes that fell on the road from a passing farm trailer. I had my worst ever bonk and had to walk the last few inclines as I couldn't ride any more. I definitely couldn't manage the out and back to the top of Inthanon. I found food on the way down and was able to roll back to Chang Mai, but was ill for a couple of days, and didn't get my riding strength back for the best part of a week (went on to tour Laos).
But ignore that as you are not doing that bit! The road to the south is much less hilly. I'm sure it would be a really memorable 600.
As we look to the new season, I am blessed by no windows for PBP qualifiers here in SE Asia.
My plans currently are a weekend at the beginning of December in the Philippines where they're running a Saturday 300 and Sunday 200.
For my 600 next month I found this devilish one in Thailand I am tempted to give a go, 12,000m of climbing though...
https://ridewithgps.com/routes/41172784