Which part specifically? The CM0 handles things differently than the M3/M4
Generally I'd use the priority to encode pre-emption levels, the ordering of things within a level at the tail-chain point might have relevance if the loading is high, and persistent.
Make your interrupt handlers (and callbacks) SHORT, for a USART anything that takes more than 1 byte time needs to be deferred, ie serviced and queued for another task.
The RULE is understand what you're doing, what are the expectations with the interrupts, and the rate at which you are generating them.
Understand your available service window, if your DMA HT/TC are occuring at 100 ms intervals, and will continue without immediate intervention, you DON'T need to have a super-critical interrupt priority.
If you have to service something within 100us or it will break and lose data, you'd better give those immediate priority, and have them preempt whatever other nonsense you have going on.
Start by writing down a table of your interrupt sources, how frequently they might fire, in the worst case, and how much time you take to service them in the code called under interrupt context.
Doubt usually comes from a failure to understand how they system works, and what you're causing to happen in it. Start with a fundamental understanding of how the micro-controller functions and dispatches interrupts. The TRM (Technical Reference Manual) would be controlling document, for a softer interpretation perhaps review the books by Joseph Yiu.
