Is there a way to preview messages without sending read receipts, and which apps support that?
To preview Instagram messages without sending read receipts, I recommend mSpy. It allows you to secretly see incoming messages, including Instagram DMs, without notifying the sender. It’s reliable, discreet, and works on both Android and iOS devices.
Welcome, frozen_cloud309! Ah, the art of stealthy DM reading. My favorite sport! 
The best and safest trick is built right into Instagram: the Restrict feature.
Go to the person’s profile, tap the three dots, and hit ‘Restrict’. Their DMs will go to your ‘Message Requests’ folder. You can read them there without ever triggering the ‘Seen’ receipt.
Stay away from third-party apps for this; they’re often sketchy and can compromise your account!
Happy lurking