Short Answer: "inside", not "among".
Disclaimer: I have no formal training in the biblical languages.
BDAG does not supply a gloss of "among" suggesting that if Luke 21 reads "among" it is reading it as a Hebraism (and he supplies a list of many that take it that way):
ἐντός adv. of place (Hom.+; ins, pap, LXX, JosAs 2:4; EpArist, Philo, Joseph., Just., D 2, 6) in our lit. functions only as prep. w. gen.
① pert. to a specific area inside someth., inside, within, within the limits of (Lucian, Dial. Mort. 14, 5; JosAs 2:4 ἐ. τοῦ θαλάμου; Jos., Bell. 3, 175 τ. πόλεως ἐντός; 7, 26; Just., D. 2, 6 ὀλίγου … ἐ. χρόνου) τοῦ θυσιαστηρίου within the sanctuary IEph 5:2; ITr 7:2. ἐάν τις τούτων ἐ. ᾖ if anyone is in their company (i.e. the comp. of faith, hope, and love) Pol 3:3.—In ἡ βασιλεία τοῦ θεοῦ ἐντὸς ὑμῶν ἐστιν Lk 17:21 (cp. Ox 654, 16=GTh 3=JBL 65, ’46, 177; also s. WSchubart, ZNW 20, 1921, 215–23), ἐ. ὑμῶν is probably patterned after ἐν σοί (=[God] is among you) Is 45:14, but with Lk preferring ἐντός in the sense among you, in your midst, either now or suddenly in the near future (cp. X., Hell. 2, 3, 19 ἐ. τούτων, An. 1, 10, 3 ἐ. αὐτῶν [on the relevance of the second X. passage, s. Field, Notes 71 and s. Roberts below]; POxy 2342, 8 [102 A.D.], of a woman who keeps a supply of wine ἐντὸ αὑτῆ ‘under her own control’; Ps 87:6 Sym.; cp. Jos., Ant. 6, 315; Arrian, Anab. 5, 22, 4 ἐ. αὐτῶν=in their midst; so NRSV text, and s. Noack and Bretscher below). The sense within you, in your hearts has linguistic support in Ps 38:4; 102:1; 108:22, all ἐντός μου; s. also Jos., Ant. 5, 107, but **Lk generally avoids ref. to God’s reign as a psychological p 341 reality. The passage has invited much debate: AWabnitz, RTQR 18, 1909, 221ff; 289ff; 456ff; CBruston, ibid. 346ff; BEaston, AJT 16, 1912, 275–83; KProost, TT 48, 1914, 246ff; JHéring, Le royaume de Dieu et sa venue ’37; PAllen, ET 50, ’39, 233–35; ASledd, ibid. 235–37; WKümmel, Verheissung u. Erfüllung ’45, 17ff; BNoack, D. Gottesreich bei Lk (17:20–24) ’48; CRoberts, HTR 41, ’48, 1–8, citing PRossGeorg III, 1, 9: ἵνα ἐντός μου αὐτὸ εὕρω; HCadbury, Christian Century 67, ’50, 172f (within your possession or reach; cp. Tertullian, Adv. Marc. 4, 35), cp. Pol 3:3 above and JGriffiths, ET 63, ’51/52, 30f; HRiesenfeld, Nuntius 2, ’49, 11f; AWikgren, ibid. 4, ’50, 27f; PBretscher, CTM 15, ’44, 730–66; 22, ’51, 895–907. W. stress on the moral implications, RFrick, Beih. ZNW 6, 1928, 6–8, s. ARüstow, ZNW 51, ’60, 197–224; JZmijewski, D. Eschatologiereden d. LkEv, ’72, 361–97.**
② pert. to what is inside an area, content τὸ ἐ. τοῦ ποτηρίου the inside of the cup=what is in the cup (cp. τὰ ἐ. τοῦ οἴκου 1 Macc 4:48, also schol. on Nicander, Alexiph. 479 τὰ ἐντός=the inside; Is 16:11) Mt 23:26.—DELG s.v. ἐν. M-M.
Arndt, W., Danker, F. W., Bauer, W., & Gingrich, F. W. (2000). A Greek-English lexicon of the New Testament and other early Christian literature (3rd ed., pp. 340–341). Chicago: University of Chicago Press.
The normal usage would be to use en mesos for "among". BDAG notes that the argument for Luke using entos to mean "within" says that Luke characteristically avoids abstraction:
...Lk generally avoids ref. to God’s reign as a psychological reality
What I find more compelling is what I take to be somewhat parallel in Paul's rare discussion of the kingdom of God:
[Rom 14:17 NASB] (17) for the kingdom of God is not eating and drinking, but righteousness and peace and joy in the Holy Spirit.
If I'm understanding Paul correctly he's saying that the kingdom is not given to the saints so they can eat and drink as they please but so that they can and will live in righteousness, peace and joy in the power of God's holy breath.
The wind is invisible but you know it is there when you see the leaves move. So it is with everyone born of the wind:
[Jhn 3:3, 5-8 ASV] (3) Jesus answered and said unto him, Verily, verily, I say unto thee, Except one be born anew, he cannot see the kingdom of God. ... (5) Jesus answered, Verily, verily, I say unto thee, Except one be born of water and the Spirit, he cannot enter into the kingdom of God. (6) That which is born of the flesh is flesh; and that which is born of the Spirit is spirit. (7) Marvel not that I said unto thee, Ye must be born anew. (8) The wind bloweth where it will, and thou hearest the voice thereof, but knowest not whence it cometh, and whither it goeth: so is every one that is born of the Spirit.
[Luk 11:20-22 ASV] (20) But if I by the finger of God cast out demons, then is the kingdom of God come upon you. (21) When the strong man fully armed guardeth his own court, his goods are in peace: (22) but when a stronger than he shall come upon him, and overcome him, he taketh from him his whole armor wherein he trusted, and divideth his spoils.
In a way, the kingdom of God is reduced to a verb. What you see with observation of the kingdom of God is the saints and their behavior in the power of the divine breath and in the teaching of the apostles.
There is another aspect of the kingdom involving togetherness, separation from sin, seeing face to face, etc. that we see referred to here:
[1Co 13:12 ASV] (12) For now we see in a mirror, darkly; but then face to face: now I know in part; but then shall I know fully even as also I was fully known.