drowning in customer messages across 4 platforms

I sell on shopee, mercado livre, instagram and my own shopify. every single message goes to a different inbox and customers expect a reply in like 10 min or they buy from someone else. I've tried the official unified inbox stuff, all garbage or stupid expensive for the volume I do. just want one inbox that pulls everything in, lets me reply, and shows which platform it came from. nothing fancy, no ai bs, just one screen.