Avoid BKK bank like the plague! Never seen so retarded and outdated banking. Took them 10 days to change my phone number (which is needed for online banking), after I had signed a dozen papers and given passport copies... Just one of many insane stories! Oh, and made a fixed deposit in Bangkok, could not get my money back in Chiang Mai, because I opened it Bangkok !??? 🙂
500 THB daily penalty is one thing, the prospect of being arrested and deported for being illegally in the the country (although rare but happens from time to time) the other. Food poisoning is usually a matter of 1 or 2 days !
"cannot connect to WISE so far" Is the wrong understanding ! Just initiate a WISE transfer USD-THB to Thailand and then transfer the money from your local account to the LOCAL (US) address that WISE gives you !
Register with WISE and then you can either transfer the money with the best available rates to pretty much any bank account worldwide or, also interesting, hold the money in different currencies in the so called: "borderless" or "multicurrency" account and then withdraw with their card in local currencies, wherever you travel. If you use my referral link, the first transfer up to 600 USD is free for you. Anyone else is also invited to use the link of course, for a free first transfer and cheap rates afterwards:
"Someone told me that from Hat Yai, Penang would be easier than Langkawi, do you agree?" Kind of, since you board the train in HY and step of in Butterworth and then it's a short ferry ride... But you seem to be interested more in nature so you will enjoy Satun-Langkawi certainly more. But as
mentions correctly, you should then take some time to enjoy it, 2 nights or more makes sense if you don't want to travel constantly, same applies for Penang.
"So you always have to stay minimum one nights, wherever you go in Malaysia?" That is totally unclear...! I've done a just turnaround bounce to Vientiane, no problem..., Pedang Besar however has cut down on this many years ago..., was before a popular choice...! In short: totally up to the immigration post, no rules whatsoever. Penag is certainly an option, but it's a bustling city, you can rent a bike an tour the island. Langkawi, via Satun might be easier and more nature. You can also try to stay 1 or 2 nights in P. Besar and come back..., with the train or walking over the border..., it really all depends on the mood of the immigration, but same day won't work.
"by going to Padang Besar and coming back right away" No, Padang Besar does not allow that since many years. You can however go to Langkawi and stay 1 night or to Padang Besar and stay a couple of nights.