You mention that you are using DKIM. Are your DKIM signatures being reported as valid?
If they are, that suggests that the forwarders are breaking your DKIM signatures. You may not be able to fix that since it is happening in the recipient’s infrastructure.
If you are not seeing valid DKIM in destinations that are not using forwarders, identifying and fixing whatever is invalidating your DKIM signatures is likely to improve DKIM survival in forwarders.
