My question is-- what platforms and technologies should I be using?
Unless you're operating at a scale where tech is the bottleneck (and by the sounds of it you won't be) it doesn't matter. There is no "should" here. Optimize for the dev experience and for writing robust, bug-free code. I'd recommend TypeScript and FireBase, with React Native for the mobile apps, but only because that's what I trust for doing the sort of thing you're talking about. If ten more people post you'll get ten more recommendations.
Unless you're operating at a scale where tech is the bottleneck (and by the sounds of it you won't be) it doesn't matter. There is no "should" here. Optimize for the dev experience and for writing robust, bug-free code. I'd recommend TypeScript and FireBase, with React Native for the mobile apps, but only because that's what I trust for doing the sort of thing you're talking about. If ten more people post you'll get ten more recommendations.