Do you work with a few specialist with 1 main contractor.

Or do you just leave it to the main contractor/ID?

a few, provide u have the time to entertain each and every one of them. of course, it saves alot of cost.

why will this save a lot of cost ???

A few if you got time to manage them !

To one if you got no time (only weekend got time).

All to one you will get more bargain power and a better quote yet the reno process will be more faster !

I did my 3 rooms hdb resale flat 10 years ago by sourcing respective contractor on our own. We save quite a fair bit.

Things that I feel easier to manage:

Window glass & grills, painting, wardrobe - get respective specialist. Cheap and no hassle.

plumbing, tiles, kitchen cabinet & electrical:

would be easier to get a good contractor to handle unless you know very well what to do.