If you translate it literally, "per" is closer to "for".
If you don't translate it literally, I'd vote for "in itself". "In itself" (viewed in its essential qualities; considered separately from other things[0]) has a different meaning than "by itself" (alone/unaided). And to me it's clear that "per se" pretty much universally means the former.
A less literal translation like "essentially" or "in essence" is deployed by master Latin translators like Robert Fagles. I've even seen "in a vacuum" which does a better job at communicating the original intent than a string of cryptic prepositions.
That's another valid translation for the same preposition.
And there are many definitions of English "for" as well. This would fit the one used in the phrase "if not for this, ..." In other words, for itself = by virtue of itself, through the existence of itself.
Also note in terms of Indo European roots, per is a cognate with English for.
Prepositions are some of the least translatable bits of language. For that matter, even without translation they tend to get slippery within a language, especially over time (one that springs to mind is the whole “quarter of” referring to a time which I first encountered some 50 years ago and still don’t know if it’s quarter to or quarter after).¹
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1. Cue some dude to tell me in 3…2…1²
2. And this knowledge will promptly disappear from my brain five minutes later, sort of like the guy I knew in my 20s whose name was either Jack or Chad and to this date, I still am not sure, but I do know that every single time I called him by name, I got it wrong and it totally wasn’t on purpose even though he didn’t believe me.
I once had a Spanish teacher, who also had problems remembering what that kind of time specification stands for and I came up with maybe a trick to remember. We do the same thing in German, so I guess it translates:
Lets say you have 11:00. That's easy. But what about 11:15? We would say "quarter 12", so I guess the English version is "quarter of 12". How to memorize, that this is 11:15? Well, you can imagine a round clock and the minute pointer has moved _quarter of its way to 12_. So you only have a quarter of that hour "already done". 10:30? We say "half 11". So I guess English is "half of 11", meaning that the minute pointer has moved half the way to 11.
Maybe this will help.
(Actually I personally usually don't use those ways of specifying the time, neither in English nor in German. I just say the 24h format as it is written: "11:15" is "eleven fifteen", 13:35 is "thirteen thirtee five" not 1pm something.)
Whatever "quarter of 12" means in English, whether it's 11:45 (quarter to 12) or 12:15 (quarter past 12), it definitely isn't 11:15. We do fraction of an hour forward or backward relative to the hour mentioned, not fraction of an hour elapsed in approach to hour mentioned.
I recently encountered a German asking for the English phrase equivalent to bis unter, looking for a phrase like "up to below". There isn't one in common use. We just don't count things in equivalent ways.
This is not great advice - the only way I have heard it in English would be "quarter past 11" to mean 11:15. Most people would just say "eleven fifteen".
As is often the case, Randall Munroe has already delivered: https://xkcd.com/1602/. Perhaps the joke in this context would be if it said "bi-weekly".
"You should come to our Linguistics Club's bi-weekly meeting. Membership is open to anyone who can figure out how often we meet." (I mean, you have a 50-50 shot. I wonder if there's any personality insights one could learn from such a selection.)
Tok Pisin, a.k.a. New Guinea Pidgin, has exactly two prepositions: bilong, which means "of" or "from" in a possessive or attributive sense; and long, which means everything else.
I have to admit that I was a bit surprised when my ex-wife listed off Spanish prepositions to discover that it excludes a lot of words I would have thought were prepositions but Spanish considers adverbs and only become prepositional when used in conjunction with one of the enumerated prepositions, usually (always?) de.