@Iris **********
I'm going to disagree, just a little bit, with the advice Maxim gave you. I think you should also consider Old City, yes it caters to the backpackers but there are also some nice quieter places for those looking for more comfort. Maybe consider north of Tha Phae gate area or around the night bazaar as well. With the noticable increase in tourists businesses are opening again and there are more places to go.
Nimman is really busy at the moment and getting busier daily. There are also a lot more flights, so plane noise is a factor.
After BKK, I'd do 4-5 days in Chiang Mai, head to Chiang Rai and possibly Pai for a few days as well. Then I'd head south to the Krabi area and travel around the islands for your beaches. Can't speak to Hua Hin as never been.
With doing visa exempt and this being your first time, hunkering down for a month will really limit your exposure to all that Thailand has to offer.
When you head to Malaysia, I'd suggest spending some time in Penang. Been reading lots about it as an excellent retirement option but haven't looked into the details. At least as Canadians we get 90 days entry so easy to take your time.
Have a great time!