Book direct with the airline or if you use an site like cheap flights, make sure all segments are with a code share airlines , then when you first check in the airline cba check you bag a the way through. If you use 2 different airlines that are not part of the same group the 1st airline will not be able to check.your luggage onto the next flight.
If you want to do a long layover book using multi city, it is very easy to have a night stop.over then carry on, I do this all the time.
Your only mistake was you were arriving and currently there is only a 30 day visa exempt entry so when you said 60 days, you don't qualify for the visa exempt so in principle you would need an actual visa to enter for 60 days. Yes it has been discussed the visa exempt will change to 60 days, we could see it July 1st but no official confirmation yet.
not true. Promptpay is becoming open to other counties, as an example I can use my Singapore Bank account to and send to a thai promptpay, and there are no transfer fees, just the Singapore banks does the currency conversion and sends the amount in baht., same thing with Malaysia.
Not sure if stillnthe same but last time I went to the bank the day before got statement and letter next day went to immigration and they said I needed the bank statement with a transaction on the same day so had to go back to back and get new letter and new statement