I'd argue that it should be an action regardless - because if it's not, then you're undermining features and perks that specifically let professionals do it more efficiently, such as Fast Hands from rogue (an already marginalised perk), letting you take the Use an Item action as a bonus action - this perk is ideal for coating weapons, which would normally be the "use and item" action.
Maybe. I think that any consumable's action use should depend on it's strength & cost (by default, all items unless otherwise specified fall under the Free Item Interaction action category...which I suppose then defaults to an action because you've already used your free-item interaction to draw the potion). A quick-coating consumable that costs a significant amount and/or deals little damage wouldn't be terrible as an item, compared to the more-common action-use coatings. Potentially Larian could even allow Fast Hands to use such quick-consumables as a free action instead of a BA (assuming Larian implements the 1x Free Item Interaction restriction of course).
But yeah I do see where you're coming from, especially with the strength of dipping in BG3. +1d4 damage takes ~4+ attacks to start having positive worth assuming non-magical items and basic attacks. This is on the edge of being worth it for fighters, so making +1d4 damage dipping a BA is probably too strong.
