Not really sure why you would deploy solo and then deploy your fleet after that. Especially when it leads to wasting PPT luring ships away and retreating from fire.
It's just a difference in challenge paradigms. Looking at it from your typical vanilla multi-ordo fights, I agree you wouldn't, and in those kinds of fights I don't either.
It's a small fleet challenge thing that happens due to leaving the 400 total DP deployment limit as is. If I'm running an Aurora flagship, backed up by some Omens and Scarabs that comes to say, 66 DP, and I'm taking on a 200 DP opposing fleet, I'm likely looking at the entire deployment against my 36 DP AI ships plus my 30 DP Aurora. I can't keep those 36 DP worth of AI ships alive by myself, so I don't deploy them until I'm sure they'll survive due to their speed. If I dropped the deployment limit to 160 from 400 DP, then I probably wouldn't need to do deployments at different times and would deploy everything at once.
It is essentially the ratio of the ships on the field at any given time. The AI can handle +20% more DP (200 DP vs 240 DP on the field), maybe +50% (160 DP vs 240 DP on the field). If you've got fast frigates to capture points, it might even just be an even fight (240 DP vs 240 DP) which the AI is happy to fight at. In my experience (and admittedly with my not so great admiral skills), the AI completely fails at +200 to +260% ratios (66 DP vs 200 to 240 DP on the field), assuming equal individual ship bonuses due to officers on both sides. Especially if a good chunk of that triple DP is just as fast as your ships (i.e. the enemy Shrikes/Medusa/Furies) so your ships effectively cannot disengage. Perhaps I'm wrong about that though?
Consider the fact that in multi-Ordo hunting you are not actually increasing the density of enemy strength, you are just making it a longer fight. Peak power is like 4 Radiants on the field (or maybe 3 Radiants and 12 Glimmers), which adds up to 240 DP. That can be achieved in a single Ordo (or an Ordo plus a little bit if you're looking for 4 Radiants). I have no doubt your end game fleet AI likely can handle that without a problem. I'd be very interested to know if typical 240 DP fleets people run to grind Ordos would work against 12 Radiants on the field at the same time or even just a normal triple Ordo with the total DP limit cranked up to 1,200 DP. I guess it would come down to how well you can prevent the enemy from focusing down your ships, and using the fact the game is 2-D to limit the concentration of enemy firepower on any given ship at a time. I admit I haven't looked, so maybe there are some nice videos showing how to handle that kind of fight (240 vs 720 all on the field).
My old computer could handle 400 total DP fights, not 1200 total DP. I've recently upgraded, so maybe I should try the 1200 DP limit setting and see how it looks. My guess is I have no fleets nor the admiral skill that can handle that, but haven't tried it so I might be surprised.
SO ships are great for player flagships but you don't have to go with fast ships like that to do great as a player. The Onslaught for instance is a great player flagship. You can have the AI deal with frigates by using ships like Furies or Scarabs or other frigate killers. Not really sure why you would wait to deploy your fleet. Just seems like a waste of PPT and puts you behind on tempo. Putting more of your bodies on the field also reduces the ratio of your bodies to there, not just because there's more of you, but because it splits up their fleet. I also don't really have a problem with the AI being outnumbered. It's helpful certainly, but Reckless AI can just power through, and if I'm running a line fleet it can even be helpful in terms of AI, since it cramps your ships and makes it easier to get them to huddle in their death ball.
Oh, I have a blast with player piloted Onslaughts. Last real play through was a Hegemony run, and that was rocking a personally piloted Onslaught XIV, along with another Onslaught XIV, Legion XIV, Legion, 2 Champion, and 3 Enforcer XIV. It is great fun diving into an enemy group with it and can surprisingly hold its own 1 on 1 against a number of Radiant setups. However, I would have issues with a single Onslaught taking on 240 DP ships simultaneously, say. I don't think I could pull off what I did in an Odyssey with a single solo flagship Onslaught, for example. Or even a single Onslaught with 2 or 3 frigates. Although, I should probably try that and confirm. Still got a backup save of that particular enemy fleet.