How to add a variation to one of base game's songs using _merge and/or _append logic to make it playable in freeplay?

[MAZ12211]

I have a High remix I want to chart and make it playable as a variation called duet instead of making it a new song/entry in freeplay.
I followed the docs on how to do this but nothing happened in freeplay, so I'm still having trouble
v0.5.3

Here's my folder tree:

mymod/
├─ _merge/
│  ├─ songs/
│  │  ├─ high/
│  │  │  ├─ Inst-duet.ogg
│  ├─ data/
│  │  ├─ songs/
│  │  │  ├─ high/
│  │  │  │  ├─ high-duet-chart.json
│  │  │  │  ├─ high-duet-metadata.json
│  │  │  │  ├─ high-metadata.json

high-metadata.json has

[
    { "op": "add", "path": "/playData/songVariations/-", "value": "duet" }
]

Here's the tool I used for making that tree folder if somebody knows where I'm meant to place those files

[AbnormalPoof]

I'm pretty sure it's because you are merging to high-duet-chart.json and high-duet-metadata.json, I'm assuming you want to add those files. Try moving them to data/songs/high/ instead of _merge/data/songs/high/

[MAZ12211]

Same result, the variation doesn't exist in freeplay when trying to switch difficulties for high (only default and erect works)
I tried moving Inst-duet.ogg out of _merge (so it's only high-metadata.json in the _merge folder) but it's the same result

Here's the mod file if it's needed
random-pack.zip