I never used to have a problem withdrawing or transfer big amounts, until last month when I wanted to pay my yearly rent. I was unable to transfers 100'000THB to my landlord and got a notice to come to the bank for ID verification if the amount is 50'000THB or more.
The daily withdrawal limit is determined by yourself but the individual transaction limit is determined by the bank. If you set your daily limit to 100'000THB and want to withdraw that amount, then you just do it in four transactions. Of course that will cost you four times ATM fees too.
Before you exit Thailand, you need to get a re-entry permit otherwise (depending on the visa you have) your visa or extension of stay expires the moment you leave. That doesn't add any additional days to your visa or permission of stay.
You can stay away as long as you like, as long you return within the validity of your visa / permission of stay.
It's not a rule but a policy. Same with the extension of stay for tourist visa. The policy to extension a tourist visa is supposed to be in case of an emergency not as a norm for longer stays. So if the immigration officer feels you're abusing your privileges, the policy (not a rule) is to refuse you the extension.
It clearly says you need a visa fit for the purpose of stay and if you're on many single entry tourist visa (or visa exempt) back to back, then you obviously have the wrong visa and you should have gotten a multiple entry tourist visa which would be fit to stay as a tourist for a substantial time.