מְאֹד is typically translated into the LXX by the word σφόδρα. Like מְאֹד, σφόδρα primarily functions as an adverb. Notably, σφόδρα was occasionally used as a substantive (viz., τὸ σφόδρα) equivalent to the abstract noun σφοδρότης.
According to LSJ,1
The Greek -της suffix is equivalent to the English -ity and -ness suffixes, all forming abstract nouns. Since English doesn’t have a word such as “veryness” or “muchness,” an appropriate synonym would be “intensity.” Hence, Plato in Republic wrote,2
ἰδίῳ αὐτοῦ, ἀλλὰ ὃ μέγιστον καὶ ἰσχυρότατον εἶχεν ἐν αὑτῷ, τούτῳ ἐπωνομάσαμεν: ἐπιθυμητικὸν γὰρ αὐτὸ κεκλήκαμεν διὰ σφοδρότητα τῶν τε περὶ τὴν ἐδωδὴν ἐπιθυμιῶν καὶ πόσιν καὶ ἀφροδίσια καὶ ὅσα ἄλλα τούτοις ἀκόλουθα, καὶ φιλοχρήματον δή, ὅτι διὰ χρημάτων μάλιστα ἀποτελοῦνται
but gave it the name of its chief and strongest element; for we called it the appetitive part because of the intensity of its appetites concerned with food and drink and love and their accompaniments, and likewise the money-loving part, because money is the chief instrument
In Deu. 6:5, we could think of מְאֹד as a word that, while typically functioning as an adverb or adjective, is here used like σφόδρα as a substantive, meaning “intensity.”
Oxford English Dictionary on the word “intensity”:
1 Republic, Book 9, §580e
Liddell, Henry George; Scott, Robert; et al. A Greek-English Lexicon. 9th ed. with revised supplement. Oxford: Clarendon, 1996.
Oxford English Dictionary online.
Plato (Πλάτων). Platonis Opera. Ed. Burnet, John. Vol. 4. Oxonii: E Typographeo Clarendoniano, 1903.
Plato. Plato in Twelve Volumes. Trans. Shorey, Paul. Vol. 6. Cambridge: Harvard UP, 1969.