I've flown SQ Y ex-SFO with heavy (but not bulky) carry-on more times than I can count and never once had a problem. Counter staff far more pre-occupied with checking travel documents to onward destinations (for example, ticket out of DPS if you're doing VOA) and never once showed the slightest interest in my cabin bag. This was all pre-COVID so YMMV. As for SIN, as transit pax, you barely encounter any SQ GAs until the moment you board. At this point, your bag will have already been screened -- security's solely looking out for prohibited items, not policing weight restrictions. Now if you're on the 737-Max flight, I suppose there could be an issue if bins are full, but likely the worse that will happen is a gate-check (no charge, but you might get nagged).