Samut Prakan immigration office is hot on the TM30 so if you did not report to them within 24 hours of arrival they may fine you. Other than that it’s a good office to use and much easier to get to if you live on this side of town.
The paper in your passport that you fill in on the plan is an arrival/departure card not a visa.
You will get a 30 day visa exempt stamp on each of your 2 arrivals. And as you won’t be in Thailand for more than 30 days on each trip that’s all you need.
You can be asked on arrival for proof of address, onward travel and funds equal to 20,000 Thai baht.