I'm surprised your coach suggested you should try your luck in the absolute division when 1) you have only been doing this for 3 weeks and 2) you would give up a significant size advantage.
That's just me thinking out loud but it obviously doesn't answer your question. When I grappled against guys 50+ lbs heavier, the only thing that seemed to work was RNC's. I pulled off triangles and arm triangles but the reason the former worked was because I hooked one of their legs ala Hominick/Edwards UFC 58, and the latter may cause you to lose position. Kimura's, armbars and guillotines never worked for me, as they powered out of each one. I did gi so we weren't able to do leg locks until we hit purple. Fancier submissions aren't recommended until you become more experienced, as they usually just cause you to take significant risks in losing position, which is why you rarely see them in BJJ competitions.