I don't think you should ever consider standard awards. It's double (total rip off), you will be better off buying SIN-BCN (saver) + BCN-GRU (saver) although in my opinion, BCN and most European sectors are total ripoffs. For a 7,500 miles more, you will get to Sao Paulo direct.

Y redemption is never a good value. Just a little more, you get 3 times more space, more comfort, better food, better sleep, better experience.

Anyway, if I were you, I would do the following (subject to availability)

1. SIN-JNB (55K in J on Thai or SAA, SQ is much cheaper at 38.25K but hard to get 2 seats in this route).

2. JNB-GRU/EZE (45K on SAA)

3. Travel around south America with Gol, Pluna, etc.

4. LIM-USA-HNL (40K on UA, AC, etc)

5. HNL-NRT (87.5K)

6. NRT-SIN on SQ (34K)

Total is 260K, will be slightly less if you can get to South Africa on SQ. It's still above your budget so you may have to buy a ticket from Tokyo back like you said. The killer is the Trans Pacific part, your budget would have been enough if you omit Honolulu and fly Lima to Tokyo in one ticket with no stopover