You have a free object interaction on your turn.
Each of the features requires you to use your action. That’s it.
Now, it is important to note, you have a free object interaction on your turn:
Here are a few examples of the sorts of thing you can do in tandem with your movement and action:
- draw or sheathe a sword
- open or close a door
- withdraw a potion from your backpack
- pick up a dropped axe
- take a bauble from a table
- remove a ring from your finger
At most, a DM could rule that activating the holy symbol also expends your free object interaction, but even this would be pushing it. If the Paladin insists on having the Holy Symbol hidden beneath their shirt, then this free object interaction is what is used to free the symbol from the shirt, after which they may expend their action activating it.
Notably, this should be able to be done while holding a sword. I just tried it wearing my daughter’s Sofia the First Amulet of Avalor and a broom. I had little issue getting it out from under my shirt while holding the broom.
As written, the Holy Symbol of Ravenkind takes one action to activate, and may use up the object interaction that you can take in tandem with your action.
Therefore, your Paladin should have no issue using the Holy Symbol of Ravenkind in 1 turn.