I would like to setup a Shopify storefront (let's call it "ACME Online Store") to enable customers to purchase products from a number of suppliers/wholesalers/merchants (who also have their own Shopify storefronts). All orders will be fulfilled (delivered) by those merchants, too. Yes, the customer will typically receive multiple deliveries.
Do I need to create a custom Sales Channel app/platform to enable my Suppliers/Merchants to add their products to my online store? Am I missing anything?
You are describing exactly what an app like Duoplane does. They automate dropship order processes. This includes automating processes such as splitting and sending individual line items to the appropriate vendors, receiving shipping/tracking information, updating inventory levels, invoicing, and more. In addition, it has integrations with shipping fulfillment systems (Shipstation, Shipping Easy, and Shippo) and accounting systems (Quickbooks and Xero) to automate receiving your tracking numbers and invoices.
Best of all, their customer support is excellent, and you can get a free trial before making any commitment. Do your research to see what best suits your needs. Don't just take my word for it - check out Duoplane on the Shopify app store and read their reviews.