but my concern is about the poor SoS... I feel they will be rolled into Custodes army just like new units to get and they will never receive anything to be fielded as a proper army (the WD "list" is so lackluster)
Well yes. SoS simply aren't a proper army in itself. They are part of the Talons of the Emperor and get deployed together with Custodes. That's how it was done 10k years ago and how it's done again today. Inbetween they barely existed. They didn't even get proper supply of armour and weapons because the High Lords of Terra deemed them unnecessary for the most part. That's their lore. They aren't nearly on the same level of SoB despite the similar name and being an all female group.
Except thats not their lore, its never been their lore and there is no good reason it should change to be their lore?
They operated extensively during the Crusade era without the Custodes and have always been a separate organisation under the umbrella of the Talons (Which has never been a formal organisation exactly, just a shorthand for the Emperors personal troops) particularly in crewing the Black ships, something which they continued doing independently on a reduced scale all throughout history. Even at their lowest ebb the semi independent groups like the one Aleya was part of are depicted as having fortresses, warp capable ships and deployments in excess of a typical 40k army. Sure their gear was worse yes, but worse than literally the best gear the Imperium can provide in Auramite stuff, which lets be honest, describes over 99% of the Imperiums military
They arent on the same scale as the SoB but then neither are Custodes, or Space Marines for that matter... In universe scale matters nothing at all in deciding whether a model range exists. Custodes are the perfect example of what they "could" do, they were literally in the same space, one plastic kit, some basic FW support for that line and a basic get you by rules set for 40k and GW did a massive range expansion and Codex for them, no reason they couldnt do it again, i mean it would be nice to get the chance seeing as FW pulled most of their resin range before it was finished (allegedly).
Mini rant done
Except that it's been exactly that.
Aside from fighting together with the Custodes they have been a really minor thing with barely any support from Terra. To such a degree that they even struggled to to keep operating at all. The blackship duty is pretty much the only thing Terra acknowledged them and gave them any kind of support at all.
I haven't read of anything showing them deploying anywhere near the excess of a typcial 40k army on their own and having fortresses and warp capable ships means basically nothing considering thousands of regular traders and rogue traders in the Imperium have warp capable ships too and "fortress" is just a name and could be anything from something as riddiculously huge as a Space Marine fortress to something as insignificant as a barely functioning ruin and shadow of its former self.
Their gear wasn't just worse, it was bad. Their armour and weapons were barely more than the materian they were made of instead of having proper power weapons and power armour (not even speaking of Custodes or Space Marine levels of power armour) and so on.
I stand by my word, there's no real in-universe reason why SoS should be their own standing army on the level of other existing factions. This has also absolutely nothing to do with a model range. They have models and could easily have more while still being part of a Talons of the Emperor faction instead of being their own separate faction.