It's possible the fact that you're likely saturating any and all PD defenses with the Gryphon spam fleet is amplifying the fighter/missile payload of the Legion, as well as likely not needing as much defense. Proximity Launcher Legion does need to get up close and personal, so having the fleet on it's back foot already from flux free damage helps a lot, as well as keep it's bombers packing reapers alive longer.
Although looking at Proximity Launchers again, I always forget they exist, and am always surprised again by how good they are in terms for raw numbers. Each one is a medium slot with 500 HE DPS. So 1000 armor penetration with heavy blaster DPS that lasts for 50 seconds flux free. EMR and Elite Missile specialization bump that up to 750 HE DPS (Plasma Cannon levels of raw DPS!) for 100 seconds. Legion becomes the equivalent DPS of 5 plasma cannons flux free and adds needlers for sustained shield pressure.
I'd wonder about running into a multiple Guardian fitted Radiant, but you're likely firing from close enough and providing the projectiles your shield long enough it doesn't have time to shoot anything down.
In regards to Onslaughts, they really wants Ordinance Expertise (Elite if you can swing it). Since Onslaught has 360 OP and Legions only 260, the typical flux dissipation difference is larger than you might initially think since you can spend more OP on guns and on vents. 100 OP spent on guns translates into 200 more flux dissipation and 4000 more flux capacity.
Onslaught also does have the advantage of outranging hard flux Radiants (and pretty much anything that isn't a Gauss Cannon), so it can run up enemy flux with it TPCs and leave it's shields down (or up and eating soft flux from beams). My typical Onslaught build includes enough PD to not care about long range missiles or fighters. I also typically mount quad Typhoon reapers with Elite Missile specialization for a personally piloted Onslaught - a burn drive in and point blank salvos of 4 reapers on any high-ish flux capital will typically force a long overload, followed up 10 seconds later by 16,000 raw HE damage from the second salvo.
Of course, the reapers are only about half the DPS of the Proximity Launchers, and 4/5ths the total damage (4000*5=20,000 versus 500*50 = 25,000).
For awhile I was trying a neural linked Onslaught to Afflictor, trying to get the entropy window to line up with the reaper salvo, but felt like more effort than it was worth.
Odyssey is more about hit and run tactics, and using it's speed to survive encounters solo rather than overwhelming DPS, which is what it looks like you'd want. If you're running a Gryphon missile spam fleet, the Odyssey is not bringing as much as the Legion to the table. The medium missile placement is not good except for guided missiles, so you're likely limited to Sabots or Harpoons. It does bring half the Legion fighter compliment though, and it's missiles aren't that far from a Gryphon loadout, and happens to tack on 2 plasma cannons, but I think five Missile Specialization Proximity launchers are going to be hard to beat.
I will note, a pure low tech fleet (i.e. not a spam gryphon fleet) properly setup does work these days, and can certainly handle anything the game throws at it.