Based on your diagram and comments, I don't see anything that would cause a burning plastic odor. It could have been a harmless "new heater" odor from residue on the heating elements from the factory.

All the AWG 14 wire is problematic and contrary to code. Under the NEC, no wire smaller than AWG 12 is allowed anywhere on a branch circuit that is supplied by a 20 amp breaker, except connecting wire internal to a UL (or other lab) approved device. For 14 AWG wire, the breaker must be no greater than 15 amps, but the way it's currently wired, a 15 amp breaker would not be allowed because your total current when everything is running is 90% of 15 amps. The limit is 80% continuous load.

When everything is running according to plan, none of the AWG 14 is currently overloaded, but the NEC plans for the unforeseen.

There are contributors here who contend that insurance companies can choose not to cover a loss caused by wiring that is clearly not up to code, as yours is now. I've never seen independent proof of that, but be forewarned.

If it's too difficult or expensive to replace the AWG 14 with 12, you might consider powering the bathroom heater from another source conveniently located in the bathroom. That may involve replacing the 500W heater with a 120V unit.

As a final check on the new install, you might power it up again, let it run for a few minutes and then carefully feel for any warmth at the wire nuts. They can overheat if not properly made up.

Based on your diagram and comments, I don't see anything that would cause a burning plastic odor. It could have been a harmless "new heater" odor from residue on the heating elements from the factory.

All the AWG 14 wire is problematic and contrary to code. Under the NEC, no wire smaller than AWG 12 is allowed anywhere on a branch circuit that is supplied by a 20 amp breaker, except connecting wire internal to a UL (or other lab) approved device. For 14 AWG wire, the breaker must be no greater than 15 amps, but the way it's currently wired, a 15 amp breaker would not be allowed because your total current when everything is running is 90% of 15 amps. The limit is 80% continuous load.

When everything is running according to plan, none of the AWG 14 is currently overloaded, but the NEC plans for the unforeseen.

There are contributors here who contend that insurance companies can choose not to cover a loss caused by wiring that is clearly not up to code, as yours is now. I've never seen independent proof of that, but be forewarned.

If it's too difficult or expensive to replace the AWG 14 with 12, you might consider powering the bathroom heater from another source conveniently located in the bathroom. That may involve replacing the 500W heater with a 120V unit. Then you could replace the 20 amp 2-pole breaker for the two new heaters with a 15 amp 2-pole. Your AWG 14 wire comes into compliance with a maximum continuous load of 76% of 15 amps.

As a final check on the new install, you might power it up again, let it run for a few minutes and then carefully feel for any warmth at the wire nuts. They can overheat if not properly made up.