Start with who hears what
The stream mix is what viewers hear. The headphone mix is what you need to hear while you stream. What your guest hears should have everything except their own voice. Give each of these mixes its own name before you add sources.
Each sound comes in once
A capture card can send its sound straight to OBS and also into a Wave Link stream mix. With both on, you get the sound twice. Remove one route, make a recording and listen for phasing or a delayed second copy. Muting a monitor output is not necessarily the same as muting the broadcast source.
Loopback is not automatically mix-minus
Loopback returns computer sound to an input. Mix-minus is a mix with one voice left out on purpose. A device can offer loopback and still need software to build the mix your guest hears. Check which channel carries what; don’t guess from a feature name.
Keep your headphones out of the stream
Headphones make the first test easier because their sound is less likely to reach the microphone. OBS can still pick up your headphone sound if the same device is also captured as Desktop Audio. Check Monitoring Enabled (Monitor and Output in older OBS versions) carefully: the device you monitor on must not be captured again.
Test the plan
Speak on each microphone separately, play game sound, bring in a guest and switch music on and off. Record the stream output and test every separate track in a player that can select it. A silent default playback track does not necessarily mean the multitrack file is empty.
For the complete connection plan, use the streaming setup builder. You can keep older equipment in your plan.