Let say i'm heading for exchange oversea for 6-8 months. Would it make sense to open a MCA and convert one shot, withdraw from time to time or convert one shot at money changer?
It probably depends on where you're going, what your spending habits are like, and your risk vs cost value.
Some countries have (additional) ATM withdrawal fees, and also cap a per-transaction limit. It could add up if you find yourself having to withdraw money often.
Carrying a large amount of cash and keeping it at home (or worse, carrying it all with you wherever you go) does not incur any additional transaction fees, but (obviously) has far more risk.
Some countries, it could be a hassle to open and/or operate a bank account, especially if you're there for a very short period of time.
Best option would probably be a combination of different things, depending largely on where you're going, and how much you plan to spend.
Change and carry some cash for your arrival and initial settling down.
Consider setting up an MCA account and use the debit function instead (there are no additional fees if you use that). Either convert SGD to the foreign currency when rates are favorable; or just keep it as SGD if you're otherwise happy with DBS's rates. Some countries are more 'card friendly' than others. (eg: Australia and US, you can practically pay for everything via card; but in Japan, quite a number of places are cash only)
If you need to charge and move a large amount, it's usually better to open a local account there, and TT the funds across.
Pro-tip: Always have a backup source of funds, and a different way to access it. Your card could get damaged or lost, bank networks could occasionally go down, or other unforeseen circumstances could otherwise make it difficult to access you money (eg. Way back, there was a period of time where it was difficult to withdraw money from US ATMs, because we had moved to 6 digit pins, but most of their ATMs would only accept 4 digits...)