Is it better to seek loan approval by yourself, visiting banks one by one, or just let the property agent or sales do it for you. Would there be different rate if DIY?

You can do it yourself if you think passing your documents to a 3rd party concerns you. Agents will introduce you to direct banker (in project case, sub-sales I'm not so sure), or outsource banker (they usually call themselves Mortgage Specialist)

Personally I will go with "Outsource banker" which they are actually insurance agents which are giving you loan application service in return for a chance to sell you MLTA. They are very familiar with every bank's appetite so you're more likely to obtain loan (and maybe lower rate), and faster processing time since they know inside out, and also have a better connection and relation with the internal bank officer as well.

If your friend or jiran-jiran is banker, can ask them to intro you their mortgage team to enquire also. It really depends on what is your comfortable level.

Are those the same with the agents in property sales?

Direct lah

They are different entity, but they usually work hand-in-hand with agents (sometimes they will pakat lahh haha, so its biased towards agent). But anything u wanna do a further clarification u can always request them to pass you the contact of the direct banker which handles your mortgage case (which is what I do, clearer picture and more neutral information/opinion)

direct better

3rd party might sway or guide you to a bank of his interest (that pays him more commission)

also to prevent your information from leaking to 3rd party misuse.

+1 use direct banker, dont use Mortgage Specialist that source from different bank and selling u MLTA.. usually they will only apply 1 bank for you and approach you to accept.

it better u have few direct banker, den u can use the offered rate to compare.

Direct banker means enquiry from banks? Or is there a separate entity/profession not under the employ of the banks? Can you give some example?

Banker that only represent 1 bank

eg:
u can contact 1x public bank loan expert, 1x maybank loan expert, 1x cimb loan expert

but not a single insurance agent that helping u apply 3 bank

my personal preference

And these bank representative would be very biased instead of 3rd party no?

Wear a Rolex and walk into a bank yourself