YYZ-HKG-SIN will get you the most miles in J and make the second leg in Y quite bearable at about only 3.5 hours on SQ vs. nearly 7 hours if you connect @ NRT. It's also quite alright upgrade wise if you fly on slower days. layover times are also quite alright with about 3 hours inbound and 5 hours outbound.

PVG and PEK are both a bit of a hassle to connect through so I'd avoid them if you are inexperienced.

AC codeshares with SQ on HKG-SIN but be forewarned that you may very well end up with a non-mileage earning fare on that leg. I also suggest pricing out UA on HKG-SIN separately as sometimes you can end up saving quite a bit of money vs the AC/SQ codeshare and also ensure you get miles for all segments.

BTW, you don't have enough credits to upgrade from cheap T+ for both AC flights unless you're crossing a threshold and have enough time between your departures for the credits to post.