Is it be advisable to apply for a credit card from a different company with better cash back rates (BOC/UOB) and then paying off the credit with my DBS credit card (so i get both the cash back from and multiplier)?

Have to check the MCC codes to see if it's excluded.

You can’t pay another bank’s bill with a credit card and get rebates for it.

Read the TnC on exclusions

It really depends on your spending habits because the cards with better cashback rates usually have other requirements to fulfill.

For the second part, technically you can't pay off one credit from other bank with another credit card...If not there will be a loophole in the system hah.

After doing some research. I decided with UOB one account as it doesn't even require you to bank in your salary. 3 GIRO accounts. But the extra effort doesn't warrant this. At max at 50k what u can get is 100 a month max including the blended deposits. Yes it's blended. Meaning the higher deposit rates are for 40k and above.

I haven't used DBS Multiplier, from what I understand, it doesn't require one to spend huge amounts on the credit card. As long as the Salary credited monthly is > 2,000. And maybe use your DBS cc for 1 purchase. That will get you near to max returns. It makes sense to me actually to charge to other credit cards to optimise returns.