If you have already enabled compressions and ok with the value changes, you can use a loop like you suggested. Loop below might be a bit simpler than your suggestion
Names Default To Here(1);
dt = Open("$SAMPLE_DATA/Big Class.jmp");
dt_summary = dt << Summary(
Group(:age),
Mean(:height),
Freq(0),
Weight(0),
Link to original data table(0),
statistics column name format("column")
);
cont_cols = dt_summary << Get Column Names(Continuous, "String");
Column(dt_summary, "height") << Format("Fixed Dec", 20, 2);
For Each({colname}, cont_cols,
Column(dt_summary, colname) << Set Values(
Round(dt_summary[0, colname], 1);
)
);
Also depending on your data, you might want to go for integers as they take less space and then you could further compress those to short-integers https://www.jmp.com/support/help/en/18.0/#page/jmp/the-shortinteger-format.shtml#
-Jarmo