Options to replace AvertX NVR

I'm working in a restaurant environment where we have a pair of AvertX NV-160s; one has nine cameras attached and 6TB of storage, while the other has 8TB and 15 cameras. Neither is providing as much video storage as we'd like--we're getting about 10 days on the former and seven on the latter, whereas we'd like to be at 30 days or more. We're working on tweaking the motion-detection settings so that we can try going to recording motion only (at the moment, it's not sufficiently accurate to set that way), but I'm skeptical about getting enough additional recording time given that many of the areas covered will have nearly constant activity while we're open, so we're considering other options for NVRs.

The simplest solution would be to buy two more AvertX units with 8TB apiece, split the existing systems across the four NVRs instead of two, and have more video retention. We have generally been otherwise happy with the AvertX units--the remote playback works reasonably well from a variety of devices (desktop, Android, and iOS), although not great for fast-forward playback; the only major annoyance I've found is the 8GB-per-export limit, which is rather unfortunate if, for example, we want to export and review a whole shift from three camera angles.

(The NVR is located with the network equipment in the basement, so we generally don't do things live on the unit itself.)

With that said, I'm looking for suggestions for other options; I wouldn't be surprised if we ended up with 20ish cameras in the one restaurant as opposed to the 15 we have now, so having a better NVR installed would allow us to add a few more cameras and still get the desired retention time. We'd like to continue using the AvertX cameras (which I believe are ONVIF-compatible) that we've already installed, as we're happy with the video quality and, well, they're already installed and paid for.

Thanks.

You can add drives if there is room or install larger drives if the units support them...

Also some versions support esata/usb external drives...call them and get details on the maximum storage limits..