There is nothing crazy about booking Jetstar. It is just a 2 hour flight

Carry on allowance

jetstar hand luggage allowance 10Kg ( 1 item plus 1 small item )
Singapore 7 Kg - One piece only

How is 1 piece of 7Kg a better allowance than 2 pieces not exceeding 10Kg?

Lowest Singapore Airlines fare is $302 with 20Kg allowance vs Jetstar's $193 with 40Kg allowance

The excess for SQ is USD10 per Kg (SGD12.53 approx) . With 20Kg allowance each, the excess to get to 120Kg ( same as the jetstar overage ) would be $1002

I would suggest that keeping luggage to within 80Kg total on Jetstar would be a good idea.

I wouldnt suggest taking SQ with $302 base fare plus $250.6 to get to 40Kg = $552 vs $193 on Jetstar

Even going to limit of 120Kg as suggested, JQ would come in at $1186 vs $1606 on SQ