Anything not installed on the router (or a server if you have all traffic going through a server) will not let you monitor bandwidth from all connected devices.
So unless you install on the router, you would have to install different applications on the individual clients, or at best install a traffic monitoring server on one device and agents on the other devices to collect the data and send to the server. So not a practical solution at all. Last option would be using packet sniffing as man in the middle, but that's problematic and also resources intensive.
The simplest solution is often the best, and that applies here. Monitor the traffic at the router.
Edit. I see now there is a package of netdata for openwrt.
It is installed on the router, but you monitor down the pipe on any browser; there are add on bits to get more data down the pipe, but have never investigated what may be available.
install wrtbwmon
follow instruction from this site https://github.com/Kiougar/luci-wrtbwmon
you will need putty and winSCP
winSCP is used for file transfer (wrtbwmon) to your router
So, wrtbwmon only seems to monitor devices by MAC address - my external, router (now running OpenWRT, previously various Tomato releases) is one network hop away from my LAN, via another router, and so all it sees is traffic from the intermediary device.
But I'm not actually looking for a break down per host.
Like the O/P, I'd become used to the Tomato Bandwidth monitor - particularly the one that would update every 2-3 minutes, but show all interface Rx/Tx counts for up to the last 24 hours - and I already miss it greatly. VnStat, specifically its luci-vnstat partner, offers nothing like the easy to read graph I've been used to, nor as granular (the best it offers is an hourly chart, where I have to eyeball the values)
I assume the Tomato approach is just counting bytes, and packets, on the interfaces which is a very low impact operation (the counters are already maintained). But then storing them in a DB file for the graphing interface to access.
I've had a look through the alternatives listed at Bandwidth Monitoring with bwmon but nothing seems to offer quite what I or the O/P are looking for?
Coming from Advanced Tomato myself (which has modern and way better interface than Tomato), I am afraid there isn't any bandwidth monitoring as in Tomato. The closest you will find is YAMon. It probably gives all info but doesn't look as nice.