No idea! I spent some time debugging it, but it just does't make sense it works with cron and not with systemd. Let me know if you have any idea where to look!

I think it’s just a shortcut because of lack of experience with systemd (this is not intended to be offensive btw - if cron is already installed and you save time by just using what you know, why not?). I would be very surprised if cron could do something that can’t be done with systemd timers.

I suspected that the network wasn't ready, so I configured the service start after network without much luck. I didn't go further than that.

Systemd can be very powerful and you can even depend on individual devices being added/removed. I’m pretty sure you can even replace the udev rules with a systemd equivalent if you tried hard enough.

I saw the apt install cron, and wondered what the solution w/out cron entailed.

Because systemd does many things badly instead of one thing well :)