If you are looking for seamless integration, you need to buy into an ecosystem. An ecosystem that offers all/most of these pieces of software and integrates them well. Obviously, not a lot of ecosystems exist that offer all these pieces.
Based on your apps list (Box, Drive, Email, Slack, Salesforce), there are two ecosystems that offer alternatives to all these apps - Microsoft & Zoho (I am with Zoho). Google is also in the mix if you add apps from Google Apps Marketplace.
So there are two options.
1) Pick an integrated ecosystem and integrate missing apps.
2) Pick independent apps and integrate all of them
You clearly are doing #2 and experiencing the challenges. It is only going to get exponentially worse as you add more apps into the mix.
Based on your apps list (Box, Drive, Email, Slack, Salesforce), there are two ecosystems that offer alternatives to all these apps - Microsoft & Zoho (I am with Zoho). Google is also in the mix if you add apps from Google Apps Marketplace.
So there are two options. 1) Pick an integrated ecosystem and integrate missing apps. 2) Pick independent apps and integrate all of them
You clearly are doing #2 and experiencing the challenges. It is only going to get exponentially worse as you add more apps into the mix.