You need to be using a RS819 with 4x4TB to get 12TB of SHR/RAID 1 storage to begin to get cheaper than Synology C2 Plan II. But then C2 Plan II can be charged annually so really your price cross-over is at 18TB of dedicated server storage. I'm assuming you would use SHR/RAID 1 and not JBOD.
Then add in that C2 Plan II will de-duplicate to save you storage. And, for both Plan II and Informaniak, just remember you need to factor in the storage for each backup version. Also, Synology enables you to select a European or US storage location, if that's also an issue.
I would look into the terms and conditions for data recovery and resilience for both services: who's issue is it if hardware fails and is all lost when it does?