I've used AEON many times - I can confirm there's no 150B charge. It's not a Malaysian; it's Japanese - they run malls primarily but also do hire-purchase. Their ATMS are harder to find because it's often just a nondescript free-standing box, but there's one in most The Mall malls and places like Terminal 21/Siam etc. There's a handy ATM locator on their website.
I use a lot of ATMs and I travel a lot all over the place so I know a little of how this all works.
1) In Thailand, first off, there's the 150B charge. This is a mandated fee and it is charged per transaction.
2) Next, all ATMs in the world use an interchange network to settle debits and credits. You know this as Visa, Mastercard, UnionPay, JCB etc. The same network is used for credit card transactions, debit card transactions and ATM transactions. The interchange network sets the currency exchange rate, which closely mirrors the interbank exchange rate (see xe.com for example). There are minor variances between networks. Currency exchange rates fluctuate during the trading day e.g. there is spot rate, there are buy/sell margins. The internetwork exchange sets a rate for the day.
3) Then your bank (not the Thai bank, they don't makan any fees except for the 150B), bloodsuckers as they are, sets a margin above and over the network's exchange rate from 0-3% (typical).
4) Finally there's the question of withdrawal limits. This is set by your bank in home currency, usually a daily amount and a per transaction amount. The Thai ATM however has a physical limit of how many notes it can dispense at one go. It is usually 20 notes so 20k baht (scb, k bank). Some banks are higher: Bangkok Bank is 25k, UOB is 30k and AEON is 40k. So if you need more, withdraw a couple of times.
So if you withdraw 20,000 baht: you are paying 150baht fee + Visa(for example) exchange rate (MC and Visa publish their daily rates, but close to or beats local money changer rates) + DBS' fee (for example 3% = 600 baht) for a total of 750 baht. This represents a 3.75% leakage.
In my case, I use an offshore bank ATM which has no fees (they charge 0% on top of Visa) and also reimburses the 150Baht so that's why I use the ATM when I need cash. I usually use a credit card though because they pay me 1% rebate.