Steel / Flying
I think this typing is very interesting and is probably my second favorite right now. The Ground immunity is very useful for checking strong Ground-types such as Colossoil and Landorus-T, which can provide switch-ins and opportunities for CAP 26 to set-up Doom Desire. The Fighting neutrality to help against M-Medicham and Tomohawk is also helpful defensively. I also think an immunity to Spikes is a small perk but still an important one for a pivot.
Offensively, I don't think Steel / Flying would struggle too much. For offensive checks, it can beat Volcarona and Keldeo, and Arghonaut defensively. It can also hit many of the offensive checks neutrally, as it's only resisted by Electric and Steel, and a few of those (Caribolt, Kartana) are hit neutrally anyway. However, if we did end up choosing Steel / Flying, I think we would end up focusing more on punishing Doom Desire switch-ins with teammates. A lot of the switch-ins offensively are very exploitable. Heatran, Magnezone, Kitsunoh, Krilowatt, Naviathan, and Excadrill (couldn't really win vs us anyway) are
all weak to Ground-types, which aren't exactly rare to find. Furthermore, most (except Kitsunoh) are weak to Fighting-types, and a few (except Heatran + waters) are weak to Fire-types, such as Heatran, Smokomodo, and Volcarona.
Steel / Water
Steel / Water is interesting because of Scald. Scald pretty much pressures every physically offensive switch-in to Doom Desire. However, this puts us in a weird place because the main thing Water STAB pressures, Fire-types like Smokomodo and Heatran, are also immune to burns, so there's not a whole lot of overlap. I suppose you could argue that this a good thing; however, I don't think Scald is enough to justify losing to pretty much every specially offensive DD switch-in, including Magnezone, Krilowatt, Volkraken, Keldeo, and even M-Charizard-Y. Naviathan gets a special mention because not only does it completely resist Doom Desire / Scald, but it also benefits off of being burned and is thus a solid check to Steel / Water CAP 26 as well.
Steel / Electric
If the argument that "Volt Switch would move Doom Desire out of the picture" is accepted, then I don't really think this type should be considered. Even though covering Waters is nice, I don't really feel like it justifies having a 4x weakness to Ground and a retained Fire and Fighting weakness, because that wouldn't make for a very good pivot. However, I do think we could potentially use Volt Switch and still have Doom Desre in the picture, as long as we don't go crazy with coverage. Either way, I would say
don't use this type if we don't use Volt Switch.
Steel / Ground
No, Steel/Ground gives you a good reason to run Flash Cannon/Earth Power/Hidden Power/ Support Move as a movepool.
To start, I want to address this. This kind of logic does not really make sense as it could be applied to literally any type. You say later that Steel / Water would not use Flash Cannon / Scald / Hidden Power / Support Move because Steel / Water does not have good offensive synergy. However, this does not seem to be the case. First of all, Water is probably the type that is most likely to run two STABs: Scald and a more powerful Water move, usually Hydro Pump. But secondly, and perhaps more importantly, any type could potentially have "reason" to run Flash Cannon / STAB / Hidden Power / Support Move. Since we are designing CAP 26 specifically around Doom Desire, this is a concern I find less effective, especially since we have many more stages to go until our final product. (We could even forgo Flash Cannon if that is a serious concern, but that's more suited for later in the process).
Anyway, Steel / Ground is my current favorite type for now. Its offensive potential is simply unmatched in terms of super-effective coverage, and, since it's already been discussed how good the STAB combination is, I won't elaborate on it too much. However, I really don't think this type would be too overbearing. It has a few defensive checks, notably drawing in Celesteela, Zapdos, Pelipper, Slowking, Suicune and a few more. Considering these mons aren't scarce, we would definitely have a healthy amount of defensive checks while beating the majority of offensive checks, thus fulfilling the role we were designed to do. Plus, I think someone has already mentioned Excadrill as an example of a Steel / Ground mon that is not overbearing in the slightest, and although they aren't exactly the same, I think it shows pretty well that we can create Steel / Ground coverage that does not overwhelm the metagame.
Steel / Ghost
Steel / Ghost has grown on me a bit since I made my initial post on the matter; however I don't think the concerns about Knock Off and Pursuit are being overblown by any means. Now that I've thought more about it I actually agree with SHSP that the problem is more Knock Off than Pursuit. Knock Off is everywhere; you would be hard-pressed to find a viable CAP team without at least one Knock Off user. Furthermore, Knock Off is a big problem if we don't run Steelium Z, because we either lose an Assault Vest or some other useful item which would give us more opportunities to set-up Doom Desire. If we do run Steelium Z, then we have less switch-in opportunities and thus limited opportunities to set-up Doom Desire. Furthermore, by setting up Doom Desire, we risk letting in a Pursuit user and then subsequently being Pursuit-trapped. I suppose this is a mindgame, but it all could easily be avoided by picking a non-Dark-weak typing. It also doesn't help that Steel / Ghost gets trapped by Pajantom, a further strain on CAP 26's role as a pivot.
I'm not arguing that Steel / Ghost is not a good defensive typing, However, I think if we want CAP 26 to be a viable Doom Desire user, we should commit to a clear direction for our Pivot. Steel / Ground clearly thrives off of offensive coverage, Steel / Flying clearly thrives off of multiple switch-in opportunities, and Steel / Water has Scald to work as a decent pressure tool to offensive switch-ins (and Steel / Electric has Volt Switch but I won't go into that right now). Steel / Ghost doesn't have any of that same direction. It doesn't thrive off of offensive coverage because neutral coverage isn't nearly as strong as broad super-effective coverage that Steel / Ground or even arguably Steel / Electric brings. It doesn't really have multiple switch-in opportunities not only because of Knock Off / Pursuit but also a weakness to Fire and Ground (and a less common Ghost) stacking on top of the prevalent Dark weakness. And unlike Steel / Water, it doesn't have a spammable and useful STAB that pressures physical attackers. All in all, this isn't the "Goldilocks zone" we're looking for.