Require few clarifications before can comment:
1) Have u guys married?
2) Do u own a house in SG?
3) Are u two SG PR?
4) If yes, how long already being a SG PR?
5) Roughly how much are both of your montly income?
6) When u planning to go back Malaysia permanently? Your whole family or some will stay in SG?

Is your Malaysia house near SG?

There is no best way, only ways that best accommodate you and your partner and later on your children.

>>Back to topic, anyone can give idea on how to give birth of baby in SG at a very low cost?
You can give birth in private hospital in Johor, it is relatively cheap compare to SG

You may also consider to buy house in Johor to bring your parents or your wife parents to stay in Johor and help look after your kids (assuming they are retired)

Or you can register your family as PR once your wife gave birth. It is easier to get PR if you apply  as a family with your child

>>6. Don't have family in SG. Was planning to go back in retirement mode around ~45 y/o. Feasible?
It all depends on the lifestyle you want, and the things (education) you want to provide for your children. If you are able to gather around RM1 mil, you can have  around RM5k per month just from interest. Not factoring in inflation

Wahh.. quite difficult for your case.

Coz as u know, it is very difficult to rent a room in SG for a family with young infant. U need to rent the whole flat and it is not cheap. Almost half of both your income gone. Not to mentioned u need to hire a maid or put in child care center.

U can't buy BTO or resale hdb coz both of u must become PR for at least 3 years unless u convert to SG citizenship. Or u can buy condominium but it is freaking expensive.

Another option is u buy a house in Johor Bahru and both of u travel back and forth but the causeway jam is really terrible. U two need to wake up very early. U also need to find a nanny to look after your child.

The last option is u put your child in your hometown and let your parents or your in-laws to look after them. This is what most lower income do. They always travel back frequently but for your case will be very expensive.

Its good u are aware and trying to think of your future. I see some of my Malaysian colleagues here still YOLO in their mid 30's.

Children education is very important and as SG is a very competitive nation, children education also become very competitive.
If you are "competitive", there is way your money is burnt... numerous tuition and enrichment classes
The syllabus is usually more advance compare to Malaysia, get more exposure and they also emphasize alot of activities
Once your children enrolled in primary... secondary, it will be difficult for them to return to Malaysia, so if you plan to let your child study in SG, might as well convert to Singaporean to get the government benefits for his people

Seen quite many sarawakian flying to SG for work, some as young as 18 years old. There are pros and cons with whichever path u undertake.

I'll say do think and discuss thoroughly with your wife to be on your future together. Either go for PR and settle down at singapore or earn as much as possible in a period of time then come back to M'sia to settle down and set up a business.

Since both of you and your wife to be is still young(20s), do hold that thought of having a baby until u have decided where u plan to settle down. Having a baby but without both parent to take care will have impact on the child's upbringing and in most cases negatively. Not to mention our parents are getting old and taking care of a baby is far from being easy so do be considerate to your own parent.

Having a child in your early 20's in SG will be a very challenging road.

What field are both of u working in? Is there a potential that both of u would earn more in the future? If both of u go back Sarawak, are u going to find a same field job or start a business? Will the income be sufficient?

After working for 3 years and gotten married, u can consider buying a hdb BTO or resale hdb.

I suggest u to use some money to learn how to invest. Its best to invest at a young age especially in the 20's.

Unlike us in Peninsular Malaysia, we cannot withdrawl our cpf money until the age of 65 if i am remeber correctly. If we give up our PR, we can only withdrawl all our cpf money at the age of 55.

To be frank, I am also in the engineering field and the pay sucks. I also cannot make use of my engineering background to open my own business.

I am planning to go back to reunite with my family next year.

At one point or another you will have to make sacrifice.
Yes alot of Singaporean do retired in Malaysia (those that can drive and do not mind risking), but generally most elder Singaporean just stay in SG. Just check the kopitiam, park and town hall and you can see retiree enjoying themselves.
The question is what kind of life style you prefer, kampung life style, city life style?

For your reference, I travel daily between MY and SG. Move my whole family to Johor.
Children study in MY government school

Thanks for the cpf info. Sorry, my mistake. Bto is only for those Singaporeans unless one of u convert to SG citizenship.

I invest in foreign stocks (US and Hong Kong). U can open an trading account in banks or trading firms. I basically open in iOcbc Security. For tips, i suggest u go youtube and listen to Peter Lynch videos. It will give a rough understanding of how to invest in potential stocks.

It's damn tiring. But this is the price you pay to enjoy SGD1 = RM3 and having your family close to you.

I joined a car pool, drop me at mrt station in singapore and pick up from some gathering point near my my house.
Can take rest in the car.
I'm not expert in trading and investment... maybe you can get more information in the investment thread.
I think public security in JB is normal    besides, you are a Malaysian right... JB is just like any other city in Malaysia... you need to be careful with your belonging and surrounding, that's all. If you compare to SG then yes, security level is different, but as Malaysia, you should do fine, we already heighten our senses
As for staying, just stay in gated and guarded area.

True too. There are pros and cons. I just pity those children who travel to SG to study in a daily basis.