Linking the systick and RTC could cause ticks to be lost on some occasions when
the RTC and systick become desync. We don't care about having them synced but
we care about their individual accuracy. Switching to independant timestamps,
using the tasks ones, fixes that and saves space
In the end, priorities are only usefull in preemptive systems. Here, it would
only garentee the execution order, which we don't care about most of the time
