There is only one, albeit huge, terminal in Suvarnabhumi. All domestic flights are located on one side of the terminal, you cannot avoid walking from the international end to the domestic end. It is more convenient to book the 2 flights together with the same airline all the way to Chiang Mai (even though your airline may use a code share arrangement for the Chiang Mai flight). Otherwise you have to retrieve your luggage in Bangkok and re-check in. When you have a single booking everything is taken care of by your airline, much less hassle.
You need to leave after every 90 days stay if you have a non O multi-entry visa applied outside of Thailand. You only need to report your address every 90 days if you have a visa extension obtained in Thailand.
Chiang Mai immigration office stopped requiring filing a new TM30 on arrival, if your address has remained unchanged. I arrived 2 weeks ago and asked the question.