I went solo with GAP Adventures to the Galapagos Islands and was very happy with the tour, but I am far from an expert about tours. (The only other one I ever took was to Russia 25 years ago when it was the only way you could get a visa)
I also spoke to some people at the Amman airport who loved their GAP tour of Jordan.
I am considering the GAP tour of Iran when Iran is granting visas on UK passports again.
However, I have read complaints on the Forum.about the level of accommodation. I don't know if the problems are real or a question of unrealistic expectations.
GAP tours are IMO often remaraby inexpensive (they were by far the cheapest for the Galapagos of all the "small" tours) and the lodging is very "local" .
On the other hand, this probably brings down the average age of the participant. On the Galapagos tour the greatest number of the participants were late twenties to early thirties, although the range was from 20 to late fifties, with a mixture of solos, couple and friends. I was made to feel very welcome.
Many had also taken other GAP tours in South America and had been pleased with them.
IMO, you probably don't need a tour in Thailand, but there is so much to see in China that a tour could be useful, but each person draws the line in a different place, so go with what makes you feel comfortable (or manageably outside your comfort zone).