Adding capacitors might do something, in general it does not fix startup issues. Can fix running voltage issues. Startup is a heavy load, and larger capacitors are also a load..... And, the startup load fouls up the effect that capacitors can provide (the heavy load lowers the "Q" of the circuit)
Since you have only the generated leg dropping very low, your issue is that the RPC cannot hold up the voltage. It's not wiring, or ALL the voltages would be low.
I think you said you replaced the motor with a WEG. What was the original motor? WEG motors are somewhat notorious for being pigs and drawing a lot on startup.
Options: Larger converter, sure. Also running another 3 phase motor unloaded to act as an auxiliary idler. That will be similar to what a larger converter would do. It would be at least a helpful test. Downside is that another motor also draws current as well as adding generated leg current capacity. But so does a larger converter.