you need a TM30. Even those like me staying on a let's say long stay visa need to be reported TM30 once they came back from abroad. Chiang Mai immigration wants that.
No visa needed. Getting in Thailand on visa exempt gives you 45 days each entry. In Thailand you can extend it with another 30 days for 1900 baht. You can do this proces multiple times a year (via land border 2 x) So not worry about a visa.
One lesson from this. Always get a re-entry permit. I never leave the local immigration without a valid re-entry permit. Its only 30 minutes more and 1000 baht for a single re-entry permit (3800 for a multiple). Better safe then sorry.
Maybe you can concider to come back on a visa exempt and do a incountry Non O request. It has some advantages such as no mandatory health insurance. But it depends on your personal situation.
Check the requirements at your local immigration. There are differences depending on the immigration. Example; TM 30 is a demand at many immigrations for many years. Some did not ask and might still not ask it.
It depend on the immigration. I did read that some immigrations accept only request within the last week. Its always best practise to check the local requirements first. With 70+ immigrations there are many differences.