If you're looking to boost damage, the best way to do it is by boosting damage. To address your specific choices:
- Alert will slightly boost your damage by letting you deal damage when you would normally be surprised.
- Charger is mediocre at best before level 5, (which is one level away!), and just plain sucks afterwards. You have better things to do with your action and with your bonus action.
- Heavy Armor Master will boost your damage significantly, since it will increase your Strength score to 16, giving you +1 to attack rolls and damage rolls. It will also help you survive, which, in the long run, will increase your damage a bit.
As you've probably noticed, there is not a lot of feat support for sword-and-board warriors. Most of the damage boosting options go to 2-handed builds. One option you didn't mention is Shield Master. It allows you to knock foes prone with your bonus action, which can help your attacks hit, thereby boosting your overall damage. But, to be perfectly honest, you're far better off getting your Strength score up to 16.
Given that getting your Strength score up to 16 is the best boost to your damage that you can give yourself right now, it's definitely a priority. So I would recommend either Heavy Armor Master, or just taking a stat increase and getting your Charisma to 16 as well as your Strength.
"Wait, what? How does boosting Charisma boost my damage?"
Well, it doesn't directly boost your damage. But neither does Heavy Armor Master. And while in the short-term, Heavy Armor Master will make you a lot tougher, as the game goes on 3 damage will become less and less significant, and more and more monsters will bypass Heavy Armor Master anyway.
On the other hand, Charisma helps with almost everything else you do - your spells, your Channel Divinity, and, from level 6 onward, all of your saving throws and all of the saving throws of any ally standing near you. Where Heavy Armor Master will get less and less important, this will get more and more important.
So, final recommendation: Either take Heavy Armor Master, or take the ability score increase and get both Strength and Charisma up to 16. They're both good options, and they'll both boost your damage roughly equally. The main difference is that Heavy Armor Master is much better in the short-term, and Charisma is much better in the long-term.