For hotels, recommend using Agoda. I have a Thai friend who works support for Agoda working from his laptop at the gym I go to. See him most days as I bring my laptop also for play. So for me if I have an agoda question I can ask support in person. But if you want private condo instead for longer length of stay for cheaper try Airbnb
Some people are over thinking it here but yes the no need for a visa and use visa exempt is most simple. You will fly in and then 5 days later fly out to your next destination. This trip will be your onward ticket requirement sorted - just make sure the 5th day trip out isn’t a land border crossing. And then as others have said when you come back you can get a 30 day extension to make up to your 5 weeks before you fly home. The single entry tourist visa might not work that well as the immigration officer may activate it in your 5 day trip rather than the 5 week one (unless you apply for one later).
Matthias Seiler TM7 form is the most time consuming document you will need filled out. Use a BLUE pen. Download it from immigration website and fill it out before you come. You will most likely need the TM30 screen shot from your current hotel or residence so make sure you get it from the property owner. Can been a screen shot on your phone. Other than that bring your passport and the cash. Come an hour early before your booked appointment time and ask the immigration office for the required forms - they will likely depending on which office you are at give you additional declaration forms to sign. You then need to go to the photocopy shop next to the immigration office and ask them to print out everything else you need. They will know what to photocopy out of your passport. You also can get the required photo printed by them. They should also have a glue stick to glue the photo to your TM7 form. Tip don’t use too much glue to avoid ink smudges going through the paper. Have I missed anything out? No accomodation receipts needed for tourist extension - just your matching TM30 address on your TM7 form. Go back to immigration and tell the officers at the appropriate counter that you have an appointment when your allocated time starts.