It does not seem exegetically or philologically sound to understand MORFH as "substance" or "nature" in Philippians 2:6-7. Jesus had the "form of a servant" insofar as he outwardly appeared to be a servant. Indeed he was a servant, but he was also much more since the disciples addressed him as Teacher and Rabbi. His outward appearance (form) did not tell the whole story: EN MORFH QEOU and MORFH DOULOU both refer to outward appearances.
Robert B. Strimple (in the Westminster Theological Journal) openly relates that for years he tried to maintain J.B. Lightfoot's strict distinction between MORFH and SXHMA until he had to admit: "there is really little evidence to support the conclusion that Paul uses MORFH in such a philosophical sense here [in Phil. 2:6ff]" (Strimple 259). Strimple also cites the four instances where MORFH occurs in the LXX (Septuagint). He writes: "in each instance . . . MORFH refers to the visible form or appearance" (Strimple 260).
It is also worthy of note that Aquila employs MORFH in Isa. 52:14 to describe the "outer appearance" of the Messiah. Since, as Strimple concurs, the theme of Jehovah's Suffering Servant undoubtedly played some part in the Philippians account--it seems reasonable to assume that MORFH as used in Isa. 52:14 bears the same meaning in Phil. 2:6ff. Strimple concludes: "meager though the biblical evidence is, it is sufficient to make a prima facie case for the reference being to a visible manifestation" (Strimple 260).
ὃς ἐν μορφῇ Θεοῦ ὑπάρχων οὐχ ἁρπαγμὸν ἡγήσατο τὸ εἶναι ἴσα Θεῷ (Philippians 2:6, Nestle 1904)
A Link for Strimple's WTJ Paper: https://faculty.gordon.edu/hu/bi/ted_hildebrandt/ntesources/ntarticles/wtj-nt/strimple-philip2-wtj.pdf