I know the reason the AI is so passive is because I've started to take on fleets that are comparatively equal or stronger than my own.
When you say comparatively equal or stronger, how much stronger? Could you give a bit more details?
For example, total DP in your fleet, total DP of an opposing fleet you're having AI issues against, and how many individual ships on each side, or even what the bounty value was, like ~300k from the intel screen assuming these are bounty fights? Like, 120 DP, Onslaught with 5 Scarabs and 5 Tempests up against 100,000 credit bounty fleets, or up against 300,000 credit bounty fleet (and whether it was a bounty from a contact or not).
Sometimes, simply having more bodies on the field, even if not individually on parity in terms of DP, can have a significant effect on being able to flank, and thus being willing to engage.
Also, could you give an example AI ship fitting you are using? Sometimes damage type mixtures can make ships more or less brave depending on opponent. Having a higher amount of kinetic damage compared to explosive damage will tend to drive flux up earlier all else being equal, leading more buildup of hard flux on enemy ships faster, which will sometimes make them more willing to engage. Still want some HE mixed in there, but in terms of just holding the line, more kinetic will tend to help.
I also agree with winning the range war via ECM suggestion. A 10% range advantage can be quite useful for improving AI behavior. Range can also be affected by other fitting choices (Safety Overrides, Integrated Targeting Unit being an option, Hypervelocity + Heavy Mauler vs Arbalest and Heavy Mortar, etc). Ships with higher speed and longer range than their opponents will behave much better than having the reverse true.
In terms of escort suggestions for a personal Onslaught, I like having a carrier or missile ship as escort, since they can shoot or send fighters over the Onslaught. A base Legion works, for example, and seem pretty easy to come by if you can get your hands on an Onslaught. A Legion XIV is better for this if you happen to stumble across one exploring. But even a Gryphon with Squall + Harpoons or Mora + Harpoons and Longbows can be good.
For example, a base Legion can stack on 5 Harpoon MRMs, Expanded Missile racks, and Missile Specialization for 180 Harpoons launched in swarms of 20 over your Onslaught even if it's behind the Onslaught, as long as there is a ship at high flux or overloaded nearby. Add in ECCM for extra speed/range on the missiles. Combine with Longbows for some long range kinetic mix. It's a basic support ship that will do something reasonable even behind an Onslaught, and since it can't move backwards very fast, should stay pretty close. A pair of Mark IXs and some vulcans can round out the guns, or other mixtures if you want something more accurate, like a couple Railguns. Probably worth investing a pair of story points into such a ship (or 3 if BotB). Legion XIV with Squall + Hurricane is a solid long range missile support (or variations on that), plus bombers to compliment. Squall + Squall wants so HE bombers (Daggers or Cobras), while Hurricane + Squall might want Longbows.