Is TM30 mandatory for stays less than 30 days in Thailand?

Yes it is. Just follow the Thai rules. If not it might come back to bite you later on.

But no problem, if you get a booking confirmation it's fine.

Yes!!!

Even 1 day, you need to submit

TM30 needs to be reported with in 24 hrs, of you reaching Thailand, by the Hotel, Hostel or Apart Hotel you stay in.

If you use an Airbnb then confirm beforehand with the host if they will file TM30 timely to avoid issues later.

Even if you stay with a partner or friends, these details need to be updated.

If a person is to stay for 90 days then TM30 becomes an important document for 90 days immigration reporting. Same for someone seeking 180 days extension.

If staying for < 30 days then the interactions related to reporting and extensions don't exit and missed TM30s are unlikely to create issues but if you play by the rules then it rather be updated.

Having TM30 is a good practice.

Literally, Owner's task. Once you moved in, Owner must register and give you TM30.

For certain government processes, it is demanded also.

So, just do it and keep it for your convenience.

Yes day 1

TM30 means address report to government. Any place where you spend a night, the owner or hotel must report via a form called TM30.

If they don't report, that's criminal negligence from their end. If you know they haven't reported, then you are viable for any issues you face.. but it's not criminal for you.

If you're staying in a hotel or guesthouse they are obligated to file a TM30 for you. If you're staying in your own apartment or a friend's, well by law your stay should be recorded too. If you are not seeking any services from immigration then it's unlikely you really have to worry about it.

Yes. You need to report your location within 24 hours of arriving.

Hotels do it for you when you check in.