Collectd: a good place to save rrd file: For Fritzbox4020

Hello ! I have OpenWRT on Raspberry pi 4 with 32GB SD card. I posted a question about luci-app-statistics for that here about the place to save rrd files, and backup-method. And it was well taken care of: set the cron to save the backup every day to flash, and bring the newest one back upon reboot, save the backup file elsewhere every few weeks. I was lazy, so I left the backup files just there for 2 months: with 32GB it's OK.

I am setting up a similar network on Fritzbox4020. Now, unlike Raspbery pi4, it doesn't have 32GB: I suppose just 16MB, right?
And, OpenWRT for FB4020 is "all working except for USB". So, this means I can't save the files on USB. This means, if I let cron make a backup of the statistics every day like I did for Raspi, and for some reason I forget to delete the back up files for a while, or I become ill or die or whatever, the router would stop working relatively soon, right? So I need an automated thing to avoid it. If anyone has an idea/experience, I would appreciate. Perhaps having the backup emailed and delete?

Another thing, how can I actually read these backup files other than extracting them into /tmp/rrd ? it would be nice if I can somehow create a graph for a month to look at. I installed rrdtools on Raspi (another one, with RaspiOS, connected to the network) but didn't quite know how to use it. I couldn't install rrdtools on my macbook pro (mid 2012, Sierra)

I would appreciate your hints very much!

There is only one way with rrdtools: read the documentation.

It may seem confusing first, but well worth the effort.

Yeah.... I studied it last time. But I don't really know where I am supposed to write all these commands, if it's going to be in a file, then chmod 700 and ./ ??
On OpenWRT there should be also config files to show me the graphs of 1 day or 1 week etc, but I can't find them. If I can look at it, I can probably figure out what to do...