I have been using CrashPlan for about 10 months and have experienced the difficulties as well that the last two updates created however it is possible to get a client to run on a Windows PC and connect to the Docker Container.
I agree that memory usage is high especially on large backup sets. Recommendation is 1 GB per 1 TB or 1 million files. I would expect that the real-time monitoring of files is what’s causing this. I have not seen any indication of memory leaks on my system.
Alternatives is deepening on your needs, is it an offsite storage location or a backup service?
I haven’t seen any real alternatives to backup solutions that can run on then unRAID OS or in a container that CrashPlan can.
Currently I have installed CrashPlan on an Ubuntu VM on my unRAID system. I have only experimented with it for a week but it looks promising. The setup is not difficult but there are a few tweaks needed to get it up and running.