Cadence change typically should not have a large impact towards ERG mode.
Can you instead design a workout that’s like a Ramp Test and see if the values match?
eg:
Target Power - 1min - 50w
Target Power - 1min - 100w
Target Power - 1min - 50w
Target Power - 1min - 100w
Technically you should be able to see the Power from Trainer match the power from your assiomas.
I think this CIQ app supports your Garmin. It allows you to “side by side” record 2 power data into 1 single session. I think you can then review the data within Garmin Connect to see if all matches up.
https://apps.garmin.com/en-US/apps/70f1aea3-f1d3-47f6-b7aa-23f029a75e3b