Merge ERA5-Land and ERA5 coastal cells

ERA5-Land contains finer-scale information of the land surface component than ERA5.  The land-sea mask used to mask out sea, is a fractional land coverage raster map.  Cells with a fraction of land < 0.5 are not considered to be land (see attached map below, red shows cells with a land coverage between (0, 0.5)).

In ERA5-Land data, what is the recommended way to fill in all those coastal cells which are not considered as land  (land coverage < 0.5) with ERA5 data?  I can imagine various solutions, yet it would be nice to have some recommendation if experience is already there.

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