Actions records my transformation in percent overrides cm

You need to be careful recording Transform in Action.  Size change will always be recorded as a relative side percentage change this is normally resizing. However the translate will be recorded in ruler units setting normally  however when I set unite to CM if records as inches it may be due to the document resolution is  set to PPI not PPC.  The position is also recorded in ruler units. Angle of rotation in degrees and if you also warp  in transform the transform step records many point  in pixel values  many points  are recorded  in Pixels not all will be displayed in the action step in the action palette.    Transform is powerful and very complex. You need to read what is recorded to know how the step will work.

Are you proportionally scaling or distorting only the height to 21cm?

If you turn the layer into a smart object, then edit the smart object, you can use image size or fit image, which does work in px and not %.

Or you can duplicate the layer to a new file, resize or fit image, then drag or copy/paste it as a layer.

A custom script may be another workaround.

This script from @jazz-y will create a smart objects from the original top-level layers (perhaps work on a duplicate of your curren doc). Then each SO layer can have an action recorded to open the SO and resize the smart object using image size or fit image, saved and closed to updage the layer content in your layered file.

/* convert all top level layers and layer sets to smart objects AM CODE.jsx

by jazz-y (DmitryEgorov)
Smart Objects in separate layers
https://community.adobe.com/t5/photoshop/smart-objects-in-separate-layers/td-p/11137197
*/

#target photoshop

s2t = stringIDToTypeID;
t2s = typeIDToStringID;

(r = new ActionReference()).putProperty(s2t('property'), p = s2t('hasBackgroundLayer'));
r.putEnumerated(s2t('document'), s2t('ordinal'), s2t('targetEnum'))
var indexFrom = executeActionGet(r).getBoolean(p) ? 0 : 1;

(r = new ActionReference()).putProperty(s2t('property'), p = s2t('numberOfLayers'));
r.putEnumerated(s2t('document'), s2t('ordinal'), s2t('targetEnum'))
var indexTo = executeActionGet(r).getInteger(p);

var layers = {}
parseLayerSets(indexFrom, indexTo, layers)

for (o in layers) {
    (r = new ActionReference()).putProperty(s2t('property'), p = s2t('layerKind'));
    r.putIdentifier(s2t("layer"), layers[o].id);
    var kind = executeActionGet(r).getInteger(p)
    if (kind != 2 && kind != 5) {
        (r = new ActionReference()).putIdentifier(s2t("layer"), layers[o].id);
        (d = new ActionDescriptor()).putReference(s2t("null"), r);
        executeAction(s2t("select"), d, DialogModes.NO)
        try {
            executeAction(s2t("newPlacedLayer"), undefined, DialogModes.NO);
        } catch (e) {
            alert(e)
        }
    }
}

function parseLayerSets(from, to, parentObj) {
    for (var i = from; i <= to; i++) {
        (r = new ActionReference()).putProperty(s2t('property'), p = s2t('layerSection'));
        r.putIndex(s2t("layer"), i)
        var section = t2s(executeActionGet(r).getEnumerationValue(p))

        if (section == 'layerSectionEnd') {
            i = parseLayerSets(i + 1, to, parentObj[i] = {})
            continue;
        }

        var properties = {};
        (r = new ActionReference()).putProperty(s2t('property'), p = s2t('layerID'));
        r.putIndex(s2t("layer"), i);
        properties.id = executeActionGet(r).getInteger(p);

        if (section == 'layerSectionStart') {
            for (o in properties) {
                parentObj[o] = properties[o]
            }
            return i;
        } else {
            parentObj[i] = properties
        }
    }
}