Growing web design / dev agency - need advice on retooling! What systems work in your team?

We're a growing marketing agency that does a fair amount of custom Wordpress builds for our clients. We went from outsourced dev to having an inhouse dev, and recently we've brought in dev number two. Our tools worked when we were small, but we're having trouble now that we're getting bigger, so I thought I'd reach out to the community to see what everyone else uses.

The tools we use were mostly ones that we just organically found a need for. They are all very disjointed and require a fair amount of work keeping everything in sync and it's just becoming unwieldy with our growing team. We currently use:

• Gmail for email - obviously going to keep this.
• Drive for file sharing
• Skype for communcation
• Basecamp to organize projects and track to-do's
• Wrike to schedule everything (although it's not meeting that need)
• Zendesk to manage support
• Trello so our dev's can keep track of who's on what
• Notism for proofing and feedback

We're ready to re-tool but want to make sure we do it right the first time. We're looking into Slack for communication and Github / Zendesk for issue tracking and VC, but aren't sure how we can integrate scheduling and other project management needs with these. We've also got years of data on Basecamp that we want to have available in case we have to refer to old projects or decisions and aren't sure how that's going to fit in with everything.

Any feedback would be awesome. Our builds usually have a Project Manager coordinating everything, a designer involved for the first half, and a dev for the last half. The better integrated everything is the better, we want to stop losing time forwarding documents to clients and having to search through 5 places before we can confidently say we can't find something.