Hi,
Like Mickael said, a boolean with sphere could be use.
I had used a boolean merge to begin the corner and delete the 3/4 sphere useless.
Next I deleted the smooth part of my quarter of sphere.
I selected the object to do an offset/shell and use an flip paramater to have bigger size than the original corner.
After I selected edges to filet and have a smoothed corner.
The object is a Solid so is printable but I have two small artefacts due to filetting.

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