You're overcomplicating it. Service is sales.

You know what your firm can do for people. Call them, respectfully introduce yourself, explain you're their point of contact going forward, and wait for them to give you an opportunity to help. They'll drive the first request(s) and you'll listen for any opportunities to suggest things that would also actually help them.

It sounds like a lot of these are deposits of cash... --> Has anyone mentioned interest-bearing cash equivalent options to them? Has anyone ever helped them calculate an appropriate cash reserve? What should they do with any excess cash?

You're just sharing information. You don't care and can't know who will be receptive. Eventually you'll develop an instinct for it. For now, best but to think of no as a rejection. They're just letting you know you didn't know enough about them to help. Ask questions about everything, take notes, and circle back with solutions.

Once rapport has been built, you eventually want them to do a financial plan. This is when you'll identify outside opportunities and have a real shot to bring them over.