I don't think this will work, for several reasons.
You need to be on High Sierra to use CUDA. There is no support in any later macOS
eGPU support for MacBooks is centred around the AMD suite, basically the RXes.
You're trying to do workstation-grade output on a laptop.
EveryMac - eGPU guide
A literal and correct answer to your question is, "No. C4D render effects are inherently whole-scene," but since there are a bunch of alternative methods that will work, what we're actually dealing with here is an instance of the XY problem. Rather than tell us how you want the problem solved, tell us what the problem is and what "success" looks like, so we ...