I do want to practice swearing like a 1950s scifi hero. "By all the moons of Jupiter!" "Lunar Landings, Pop! It looks like you need some help!" "What the space is going on here?" Real words, silly context. (I already say "dang" and "heck," so some people think I avoid swears, but I just think my choices are more amusing. )
When moderately mad, I do vowel-less cusses: "fck" "sht." (I mostly cuss/communicate online, so pronunciation doesn't matter as much.) And then people know I'm really mad if I don't blank out a letter.
So be sure within your book, different characters have different levels of swearing. Most of my friends drop F-bombs casually. Some get weirdly creative. Some have specific reasons for which words they will or will not use. Some won't say more "casual" curses because they have a misogynistic origin, others are Australian. (joke -- but non-U.S. countries are more casual with the C-word, where it's highly radioactive here.)
Deadwood used current-to-2005 sexual cursing, because the current-to-1870s cursing was more religious based, and in the show would sound dainty and silly. So they sacrificed literal accuracy to the time period for emotional accuracy.
Although I said I mostly type, so I don't really worry about pronunciation, I think that fake curses need to have phonemes in common with the "real" one. "Frak" doesn't work as well, because the vowel sound of long-but-flat "a" is so different from the flexible schwa of the "u." Some novel, post WWII, (blanking on the name) used "fug" as the cuss -- I think that softening the "ck" to a "g" is worthy concession to the mores of the time. The vowel still gives the flexibility to be stretched, and the word feels like it can be used as any part of speech.
"Merde" is an odd one: Picard got away with it in ST: TNG because it doesn't sound like shit. Yet I see a connection in the vowels -- both the "air" and the "ih" have a bit of a slither -- neither word has harsh consonants -- the "t" in shit is typically glottal-stopped to a sound closer to a d.
For whole phrases, borrowing from another culture is a great way to have a coherent-enough system. In Babylon 5, the Narn greet people with "Have you eaten?" -- this is from Korean culture. (Source - Lurker's Guide to B5, I think?)
Try to extrapolate WHY a metaphor may be something that a culture originated, and then double-extrapolate to how it may have changed. "The exception that proves the rule" - prove used to mean to test/challenge something. (How do you know water is wet unless you have dry to compare it to?) But now people use it with the more common meaning of proof -- this verifies the rule is true.
So the way it's commonly used is an error, but only pedants like me care. Do you have characters who would know the origin (or think they know) of a phrase, and try to correct people? Are yours the ones who just go by what they heard (not read), or vice versa? Did they learn these phrases from classmates/peers, or from parents, or from adults they were with in an unsupervised setting?