no, it is not possible. You need to prepare the scene on a CUDA machine, even at low resolution, to be able to send it to the cloud to be rendered in high res.
If you can go back to High Sierra, even on an external hdd, you should be able to use Octane via Thunderbolt port eGPU solution:https://egpu.io/setup-guide-external-graphics-card-mac/
I personally work everyday with an iMac 27" 2017 5k with Thunderbolt 3 eGPU 1070, under High Sierra.
Please, note that we are going to introduce Octane X, later this year, to definitely solve all the Mac issues:https://home.otoy.com/octane-x-wwdc2019/