I don't believe it is possible to show the units for each value - only for the totals as you have discovered.
I often put the units in the text for the column header or as you have suggested you could use a column of custom text.
You can use superscript text if you copy and paste it from your floor plan text - or any text editor - even this post.
i.e. type 'm²' in the floor plan (or text editor) and then copy and paste it to the schedule.
Once in the schedule you can copy and paste it from there.
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