From time to time there are promotions for shopback/shopback go (eg:Guardian rebate etc) i would say there AFAIK is no harm using registering your credit card and earn rebates on shopback go. In fact you may actually find surprise cashback in shopback go when you dont expect it.

30% - 50% for now, but only if you already set out your emergency funds and insurance covered. Remember to always have a safety net before investing.

I would say go with 6 month of essential expenses and place them in high interest bank account for liquidity (eg: StandChart Jumpstart, CIMB FastSaver etc) Have another 3 - 6 months of non-essential expenses and keep those in high yield, less risk investments (eg: SSB, bonds etc) In the event where you need more than 6 months of essential expenses (touchwood) at least you will be able to moderate your expenditure based on the additional 3 - 6 months and will not be forced to a loan or liquidate your investements.

Investing early is definitely one of the things to do, but only if you can afford. Since you've mentioned money not seen is money not spent. You can probably automate this process? Perhaps setting up a regular RSP using DBS Invest saver or purchasing of UTs monthly. And slowly build up the pot. After awhile, the pot will grow and perhaps you will learn and appreciate the value of investing early.

Yes, you will earn he base rate of 1.2mpd but there are better cards out there that offer a lot more. like UOB one etc.

Many actually want to apply the card cause of the double dipping rewards. You get grab points on top of your usual CC miles/cashback.

I would save the grab points for discounted/promotional rewards. For example StashAway has $5 off for 1200 points. This works out to be 4.17% of rebates. (using 10 points earned per dollar) The same rate works out for Pezzo, Love Bonito etc. I am sure most in the miles camp claimed the 50% KrisFlyer promo in June. which lets you claim 700 Grab points to 160 Miles (750 for Silver/Gold) 2.28 MPD, on top of the miles/cashback you earned for the topup. If you rarely take grab, these are the options that are more worthwhile IMO.

FavePay, GrabPay, Paywave then nets. Simply because FavePay earns cashback on top of grab points. GrabPay pays you miles/cashback on top of the points. And paywave if the previous 2 isn't available

For online transaction Citi rewards visa with 4mpd is the one I prefer out there. But take note of the $1k limit per card statement. For dining, if you can use paywave, UOB preferred platinum visa is good with 4mpd, for this there is also a $1k limit. General spending, I will go with DBS Altitude with miles that has no expiry. Currently they have a 4 mpd for bus/MRT rides under simplygo as well.

Insurance is not counted for 1.5% cashback. You may want to consider HSBC advance, they provide 1.5% cashback too but its capped at $70 per month.