I'm just trying to outline a held item without the invisible Armor Stand getting shown. I don't think there is but it's worth a try asking. Is it possible to only let the item on an Armor Stand glow?
No. Glowing is a status effect that can only be applied to entities.
There used to be a workaround by summoning a visible regular armour stand without Glowing and item and an invisible "
Marker:1" armour stand with Glowing and item at the same position, but this no longer works. You can vote for the bug report here: https://bugs.mojang.com/browse/MC-167220
You edited your original comment so now I understand that you meant that you should make it a marker. This got fixed now in 1.15.2 pre1 like you said it would in the comments. Now I also realize that you thought I wanted the armorstand to be visible which I don't.
So answer is summon the armorstand as a marker with glowing and only the item on its head will glow.