I've been several times in recent years and absolutely loved it. Please don't be put off by PP's comments as it's really very individual, and I would hate to have missed out on seeing this amazing country. Yes, it's no doubt more touristy now than it was years ago, but on the other hand, Thailand does tourism very well, and honestly, I'm happy to share a beautiful place with other tourists who come to marvel at it. Friends who've travelled the region extensively all comment how much easier it is to find your way around as a tourist in Thailand than elsewhere. It's easy to find the most beautiful places and to arrange tours to them, but there are also plenty of stunning places where you can have the place almost to yourself. Read up on Thailand seasons - generally, the Andaman Coast side, eg Phuket, Khao Lak and Krabi, are best Nov - April. Chiang Mai has burning season in Spring where they burn crops; April is the worst time for this but iirc they start in March. Do check. If you're travelling in the rainy season (our summer) then the Gulf of Thailand is nicer - Koh Samui, Koh Tao etc. At that time of year the Andaman Sea can get very rough apparently, so you may not be able to do trips to some islands.
There are seedy areas but do your homework and avoid them - eg in Phuket, Patong is seedy but eg North Phuket, Naithon and Naiyang, are stunning beaches in areas mostly visited by families - I remember many kids playing in the sand under fairy lights while parents enjoy their evening meals. Kata and Kamala are beautiful beaches and Kata seemed very family-friendly when I visited. Phuket has great beaches generally, soft, white powder sand for the most part, with gently sloping seas.
Khao Lak has lots of space and big chain hotels and some great luxury resorts. The beaches are long and clean but they aren't quite as pretty as Phuket's, from what I saw. The area is good for jungle and adventure trips inland.
Koh Lanta is like an almost undiscovered version of Phuket - similar landscape and great beaches, but much quieter, can be sleepy.
Krabi has the stunning limestone scenery and beautiful beaches, plus many gorgeous islands to explore. Ao Nang town and main beach is very busy but there are other beaches in the area which are much quieter eg Tubkaek, Khlong Muang. You can do a day trip to Railay (or stay in Railay) and day trip to PhiPhi islands from there.
Chiang Mai is interesting for a different view of Thailand, but inland so no beaches! Personally I found it just a nice, ordinary town, with lots of temples , lots of Westerners who stay there long term as it's cheap, eg digital nomads and people learning Muay Thai. It's a long way from the great beaches of South Thailand, so I think you have to really want to go there or be primarily interested in culture for it to be worth spending time travelling there. What with checking in for internal flight, the flight itself, and general transfer-related hassle, you'll lose a day in travelling to get there. Whereas you can get between Phuket and Koh Lanta or Krabi in a couple of hours.
If you go to Bangkok, one tip which worked for me was to spend time there at the end of the trip rather than the start. This allows you to get used to Thailand so it's not overwhelming, and also means that if you're flying internationally from Bangkok, you're already in town for your flight home, rather than spending one night in a hotel in town just to be there for a flight. Internal flights can be unreliable so you need to allow plenty of time to reach your return international flight.
I hope you have a great trip and enjoy planning it. There are lots of travel blogs and YouTube channels which will show you different areas and what they're like right now.