How to bevel something like this?

[etn249]

Maybe it would be possible with the “bevel after boolean” addon:

[WIP]Bevel after Boolean

I apologize for the automatic translation. Hello everybody. I recently started writing a small script that allows you to do Bevel after the Booleans. I just started learning Python and Blender api. Therefore, my implementation is very primitive and contains errors. My code may look very bad and contain many errors. But I’ll try to fix it in the future. To use, Tools Panel>Boolean Bevel. Description of functions: Show wire - Displays and hides the wire Change Boolean Operation - Allows you …

There are multiple versions of the addon on this page.
However, It doesn’t seem to work in the current version of Blender, so you might need to use an older version.

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If that’s not an option, I have a method that will give you the look you want, but the topology will be even messier. Also, I don’t know if it’s precise enough for your case.

1-Use voxel remesh on the object. Go as fine as is reasonnable. If you used the remesh modifier, apply it
2-Use the “smooth” modifier and increase the “repeat” setting until the corners are rounded as you need. Apply the modifier.
3-Use the decimate modifier so the object has a polygon count that can be worked with. Reduce until you start seeing a visual difference, then go back a bit to give the mesh some margin.
4-If the outer corners need to stay sharp, you can extrude the outer faces before the procedure and then boolean them back to perfect flatness at the end.