As far as I know, per annum is actually a part of the English language. I typically have seen it used in the context of pricing or finance which seems to be verified. [1] I don't disagree with you about p.a. less clear than /yr though.
I'd argue "per annum" or p.a. isn't a foreign term and just standard English. English is full of Latin words, transparent, opaque, foreign, those are all words of Latin origin.
> I'd argue "per annum" or p.a. isn't a foreign term and just standard English.
Did you notice that a fluent English speaker had to ask what "p.a." meant? Do you think that would have happened if it had been written "per year" instead?
Per annum, per centum, per mille, per capita, et cetera, et cetera.
These are Latin phrases, borrowed especially in British English as Great Britain was occupied by Latin speakers for nearly 400 years -- 43 CE through 410 CE. Latin continued to be the language of diplomacy, religion, philosophy, and science through the 18th and 19th century.
It is, for all practical intents, proper British English.
> borrowed especially in British English as Great Britain was occupied by Latin speakers for nearly 400 years -- 43 CE through 410 CE.
Can you explain how Latin loanwords were loaned into English during this period? In your explanation, please make use of the facts that (1) there were no English speakers in Great Britain before 410 CE, and moreover (2) there was, by definition, no such language as English until Anglo-Saxon migrations into Great Britain (around 450 CE) established a distinct West Germanic linguistic community on the island.
Even "$500k/yr" is an improvement on "$500k p.a.".