+1 to the external clients. Not sure if it makes sense for it to be agency driven with external client stakeholders, or company driven with external agency developers/PMs. I'd imagine the former would be more common but could be quite a different solution for the latter.
At the moment, $400/mo is more than the savings calculator suggests we can save, but we're currently 1 full time, 1 part time iOS engineer, 1 release a month, 2 apps (same codebase, whitelabelled). I think $400/mo for ~10 devs makes sense, at that scale it's similar in cost to our other collaboration and CI tools, but I suspect we'd have a hard time justifying it until we're at that sort of scale.
Are there costly features you can cut that could make it, say, $100 for < 5 people perhaps? Something that opens it up to small teams who can then "grow up" on the platform?