That's a big question as there are many types of insurance. But I won't mind addressing a few.
Let's do health first as I have a few words to share. I went for health insurance from my home country, looking for quality. Didn't want to expose myself to the risk of having to put up a juridical fight here in Thailand. It was also convenient that some travel related items were covered under one and the same policy (think of a relative for whom you urgently need to get back, or you get an accident and need to get back yourself). What I overlooked is that Thai insurances have a few perks over foreign ones. My insurance premium was subject to 21% tax for instance, which would have looked better in Thailand. Also: insurance premium in Thailand is deductable from your income tax. I don't know why that's kept a secret; online sources all do mention that this holds for life insurance but to know it also holds for health you have to actually file a tax return once. The two are nicely listed below each other but websites stubbornly leave the health one out. Anyway. The deciding factor to go with a Dutch insurance is the structure of it. I didn't care if the insurance covers a few scratches or bandaids every now and then. What I do care about is if I turn out to have, you name something, jaw cancer or the like, that there would not be any ceiling to the coverage. Thai insurances pretty much all have that ceiling. So although my premium came out disproportionally high, the insurance structure would be a lot more suitable (and didn't end up getting something that serious, luckily).
Travel insurance. My expat insurance covered a few things that I found nice. Valuable items for instance (at an extra premium, but it turned out great to have with the various cameras that died in accidents). I've already mentioned flying to home country which was also nice coverage, but whether you need that also depends on e.g. how many relatives you have at high age or in bad health. If you don't have any of your grandparents still alive, and your parents aren't expected to leave yet, you may not need it. In the exceptional case that you still need that urgent flight home, it isn't gonna bankrupt you. Also factor in the premium. I've switched from having this covered (when the premium was fine) to excluding it (when there was no longer a choice to cover just myself and the threefold premium was not worth it as the coverage was only relevant for me).
Vehicle insurance is mandatory and I can't advice much. You obviously can only get that in Thailand. I'd prefer coverage over premium and would advice accordingly.
Property insurance: can maybe also be included in expat insurance if you go with that but if you don't, just leave it. Rental condos come with furniture, fire safety is on par with western standards, smoke detectors everywhere, risk of theft of stuff from your home is negligible.
Life insurance: I've always found this a stupid one unless your own a house, have a mortgage, and earn the family's income, and find that your family should stay in that house if you leave unexpectedly. In all other cases, don't get this nonsense.
Dental insurance used to be viable but today only people who are nearly certain that they claim the maximum amount buy it. It is thanks to a few ignorant donators that they still offer you a maximum coverage of about $500 at a premium of $450, so if you are indeed certain to spend the amount, you could grab the $50. If it's not be certain, don't buy. Dental accident insurance can be good though, that's a different thing.
Juridical insurance: I had it but to my discomfort it excluded labor conflicts. Now that would have been one of the major coverages for me. I'd try to get one covering that if it's relevant to you as it was for me.
Liability insurance: that's a do. Premium usually very low, and the type of thing they cover could get you bankrupt if you don't have it insured. Get this.
Well, yes. Not the whole day, but you can't expect many days without rain. If you would prefer holidaying without rain, then there are better months than August.
By the way, Bali and the rest of south Indonesia have the rain and dry season reversed. Those are below the equator.
the number of expats in Thailand is estimated as 3 to 4 million. With now between 1000 and 2000 DTV holders, many of whom would otherwise still come but on different visas, we are talking about less than 0.05%. It will of course increase because the DTV is still fresh, but eventually it would not surpass even 1%. It surely is a win for the economy and every bit helps, but we should not expect condo prices to move significantly.