Under your circumstances, I'd go for LH F.
Most of the 747s are refitted and on SIN and PVG you'll enjoy the A380 F cabin.
I guess the connection in FRA isn't too convenient with plenty of time to kill in FRA.
Check out BA F fares too: They should be around the EUR 10k mark.
By the way, what SQ C fares are you referring to ?
Business fares seem to be reasonable.
[KVS Availability Tool 7.1.7/Diamond - ATP: Fares/FC/Current]
Code:
GRU Sao Paulo Guarulhos Intl SP BR [SBGR]
SIN Singapore Changi SG [WSSS]
R/T
Carrier From To Fare Cur Total Q YQYR Fare Basis
--------- ------ ---- -------- ---- -------- ---- ---- -------------
SQ SAO SIN 5696 EUR 5696 DTA
SQ SAO SIN 5696 EUR 5696 JTA
SQ SAO SIN 6076 EUR 6076 JTA
SQ SAO SIN 6835 EUR 6835 CTA
SQ SAO SIN 6835 EUR 6835 CTA
SQ SAO SIN 8354 EUR 8354 ZTA
SQ SAO SIN 8354 EUR 8354 ZTA
SQ SAO SIN 11392 EUR 11392 PTA
SQ SAO SIN 11392 EUR 11392 PTA
SQ SAO SIN 12152 EUR 12152 CSQ
SQ SAO SIN 12152 EUR 12152 CSQ
SQ SAO SIN 17848 EUR 17848 FSQ
SQ SAO SIN 17848 EUR 17848 FSQ