As my post above on ἡ λεπτότης shows, "subtlety" is a word used by Epicurus in discussing physics -- but not (except for this example of the manuscript's version of VS 63) when discussing morals.
Among the examples, this quote below must be the best, because Epicurus stacks near synonyms, and in doing better shows us what he thought of the word:
"...It has the force to eject air, [while] carrying [the air] over – and It is evident that this same Force also exists even in the films: for if only a hard Object had the force to produce ejections ¬ but a Film did not [have the force to produce ejections] – then only hard Objects would have the force to be quickly carried in the ejecting way ¬ but the films would not [have the force to be quickly carried], at least [not] in the ejecting [way]: however, in what is completely encompassed around immediately from the ready void, through the settlements into narrowness, subtlety, and minuteness……"
[Epicurus, On Nature, Book 2, P.Herc. 993 col. 10 (column 111inf) | P.Herc. 1149 col. 5 (column 112sup)] "…[ἀέ]ρα ἐξ̣ω̣[θεῖ]ν [δυνα]τὸν περαιοῖ, φα̣[ν]ερὸν ὡς καὶ τοῖς εἰδώλοις ὑπάρχει καὶ [α]ὕτη ἡ δύναμις· εἰ μὲν γὰρ τὸ στ̣ερέμνιον μόνον ἠδύνατο τὰς | ἐξ̣ώσεις π[οιεῖσ]θαι, τὸ δ̣' εἴδωλο[ν μή], ἦν ἂν κατὰ τὸν ἐ̣[ξ]ω̣στικὸν τρ̣όπον τὰ στερέμνια μόνον ταχέ̣ως δύνασθαι φέρεσ[θ]αι, [τ]ὰ̣ δὲ εἴδωλα μή, κατ̣ά γε τὸν ἐξωστ[ι]κό[ν], κα[τ]ὰ μέντοι τὸ περ̣ι̣λαμβανόμενον εὐθὺ[ς ἐξ] ἑτ[ο]ίμου κε̣νοῦ διὰ τ[ὰς] συνιζήσεις τὰς ε[ἰς στ]ενότητα καὶ λεπτ̣[ό]τ̣ητα καὶ μι[κρ]ότη[τ]α…"
We could draw a line from "subtlety" to "simple living" if it was otherwise established, but I do not see that it is.
The closest I have is in The Double Indictment (section 2) Lucian has Zeus complain "I myself have to do any number of tasks that are almost impossible to carry out on account of their subtlety (ὑπὸ λεπτότητος)" -- which may be enough to draw it all together and preserve the manuscripts reading.