You don't say what type of rendering you will be doing. If you can't buy a new system any time soon, then I agree that Corona is your best option for your Trashcan™. I have a couple of them and Corona runs great. Of course, the faster the system the faster the rendering, but Corona can run on pretty old CPUs so you can build your own render farm utilizing older Macs. I just switched to an iMac Pro from a former coworker and have the trashcan on my network via Ethernet. Team Render works great. Corona also runs on C4D 14+.
Hopefully you have the 12-core version. The Corona devs are still adding features and fixing old bugs, but I use it every day and am very happy. It also has an Interactive Render window and you can set it to it in the actual viewport. It's not going to be as fast as Octane on a $4,000 GPU, BUT you can get work done now while you figure things out. The workflow is similar to the other engines, so it won't be a huge leap if you switch.
You can download a 45-day demo with full functions. I think it's cheaper than other options too. I think about $340/year or you can do month-to-month. I have a bunch of images done with Corona on my Vimeo page. If you have any Corona questions, let me know.