2 Cor 9:6 Τοῦτο δέ, ὁ σπείρων φειδομένως φειδομένως καὶ θερίσει, καὶ ὁ σπείρων ἐπ᾿ εὐλογίαις ἐπ᾿ εὐλογίαις καὶ θερίσει.
It seems to me that the context of the collection for the πτῶχοι of Jerusalem makes the sense of φειδομένως here clear enough. It's pretty clear that φείδεσθαι here is used in a pejorative sense of "stinting, niggardliness" where one ought to be generous. One must, of course, forget the parables suggesting that it doesn't take much seed to produce a bumper crop; the context here is different. If the farmer doesn't put down a goodly portion of seed, he cannot expect much of a crop. So niggardliness in benevolence giving won't accomplish much that's praiseworthy. So φείδεσθαι here has the fundamental sense of parsimony
and εὐλογία has the fundamental sense of benevolence or generosity.
For my part, I think that BDAG s.v. εὐλογία is right on the money
4. Since the concept of blessing connotes the idea of bounty, εὐ. also bears the mng. generous gift, bounty (opp. πλεονεξία) 2 Cor 9:5b; perh. also 5a, s. 3c above. ἐπ᾿ εὐλογίαις (opp. φειδομένως) bountifully 9:6ab (Philo, Leg. All. 3, 210: ἐπ᾿ εὐλογίαις=in order that blessed influence might be felt). This may perh. be the place for Hb 6:7 (s. 3bα above) γῆ . . . μεταλαμβάνει εὐλογίας ἀπὸ τ. θεοῦ the earth receives rich bounty from God. RAC VI 900–928. JMateos, Análisis de un campo lexematico—Εὐλογία en el NT: Filologí a Neotestamentaria 1, ’88, 5–25; cp. Betz, 2 Cor 96–99, 103.—Renehan ’85, 73f. M-M. TW.
P.Oxy.1065 ὥσπερ οἱ θεοὶ οὐκ ἐφείσαντό μου οὕτως κἀγὼ θεῶν οὐ φείσομαι (lines 7-9).
I'd need to see more of the context, but I can readily enough imagine a positive sense, "As the gods have not stinted me, so I won't stint the gods." or a negative sense of, let's say, William Ernest Hensley, "As the gods have spared me nothing in abuse, so I'll not treat them less ungenerously."
And regarding the etymology of εὐλογία, I don't think we need to suppose derivation more directly from the "gathering" root sense of λεγ/λογ. The usages cited by BDAG for εὐλογία in the sense of beneficence are quite adequate, I think.