I want the client to be able to chat with me via Slack, attach pictures or documents if necessary, and receive and send Slack messages from a specific Slack channel.
I think to help you find a better solution we'd need to understand a little more about what is or isn't working well here.
The Slack APIs aren't typically meant for building chat client-like functionality outside of Slack, so you have some natural constraints around how you retrieve message history and poll for new messages without building something more substantial with their event-driven APIs.
What are you looking to most improve in your implementation?