I'd recommend letting Steele be a bit more active in coaxing that kind of transformation.
For instance, one thing that comes to mind is a pussy-wielding Steele being able to have another bar orgy, but this one involving both holes and with Steele making an effort to fan the flames of jealousy in Del by crooning over how good it feels to have a pussy. Obviously you need an alternative method if Steele is dick-only, but that type of diegetic option is ideal.
Though speaking of dick, Del's cock is something to consider. Personally I think it fits the fetish best for her not to be allowed to use it until fully broken in, but I wouldn't mind herm or shemale Del getting a dose or two of Throbb in her to give her something up to Syri's standards if/when Steele decides she's ready for the reward of fucking the captain. For trap Del, not being all that hung is part of the aesthetic, and it's more thematically fitting for Del to only gain what many would consider a sign of masculinity once she's changed in a way that twists it instead into a sign of the superiority of the feminine form. A height bump is another good cue to use for this.
That basically leaves you with four form options, though thankfully only two scene sets and personalities:
* Trap Del: don't break him in, Del stays male and meek/cutely uncomfortable. Pretty much the same bits of content Del has right now. Kind of a different rabbit hole from the other options, I'll let someone more interested in traps outline what they'd want from that content.
* Shemale Del: break her in but don't tempt to a pussy, Del goes busty and slutty. Possibly make her taller than Trap Del on the second encounter, then either grow her dick bigger as part of the second encounter content or make it something she does between the second and third. Should probably be her default if the player ignores her on the second encounter.
* Female Del: break her in on the first encounter and tempt her to switch from dick to pussy on the second. Probably can share almost all of Shemale Del's dialogue and any scenes where she doesn't actually use her dick on someone, with adjustments to account for the switch, plus a vaginal scene.
* Herm Del: break her in, get her to both get a pussy and grow her dick. Same content as Female and Shemale Del with some extra lines regarding the extra bits. All three of these are basically the same character with the scenes adjusted a bit to account for dick / no dick, vag / no vag, so ideally it shouldn't be too much hassle.