good fantasy advice: chase targets, not touchdowns
Megatron has seemed pretty bad the first four games (27/255/1) but over the last 3 games has seen 17, 13, and 11 targets. Mike Evans did see 17 targets in the Houston game (went 3/32/0 on 8 targets vs Carolina, but Josh Norman is pretty fuckin good), but Megatron is probably an upgrade over him.
Phillip Rivers is not even a downgrade over Ryan to be honest, their passing stats are basically the same even though the Falcons' offense has been loads better (9 rushing TDs so far this season, which has been leeching some of Ryan's effectiveness). You should 100% be starting Cam anyway every week, rushing value is too stronk for QBs and it shows in his stats. besides, if you needed one week to start and play Rivers, the Steelers' secondary isn't exactly a bad target.
QBs are notoriously hard to trade because people tend not to value them as much as they do come draft time. You have two solid QBs, but every other team probably either has a top 10 QB or one that they think will improve as the season goes on (Brees, Manning, Luck) so you will be hard pressed to find a good deal. Cam is more shoppable, especially after he has his bye this week, but I don't think you will have great choices for trading partners anyway if you wait.