Hi Ryan, ok, I will start with this....
I am an Atheist because I do not believe there are any gods. I am technically also an Agnostic because I have no knowledge about any of them. There is nothing objective for me to consider, to learn from, that would convince me that my lack of belief is unwarranted.
Atheism is a lack of belief in a particular god. If you do not profess to have faith or belief in any god then you are an Atheist. Christians are atheists towards Hindu gods. I am towards all gods. I have no knowledge of the existence of any god so I am agnostic towards them all to. The evidence is completely absent. There is none, not a sliver. So I am a “strong atheist”. There are no gods.
The reason I don't call myself an agnostic is because the connotation is that an agnostic is someone who simply says "I don't know," or "It can't in principle be known." I'm an atheist because I want to say something stronger than that. When a theist makes claims about creators and insists that I'm a slave to one, that I'm owned by and eternally in debt to whatever god they're pushing, I have a right to demand evidence. They haven't produced a single shred.
In my opinion, if these people are bold enough to lie right to my face and make claims they have no business making, I ought to be bold enough to say "I don't believe a word of it." I'm an atheist.
To capitalize on your Santa analogy, you wouldn't just say "I don't know if Santa exists," if someone really expected you as a grown adult to believe he did. Saying that you didn't know would be beneath you. I've never been to the North Pole, I can't prove that toy making elves don't exist, or that magic reindeer can't fly but I can say that there is no credible evidence that they do exist. Anyone who wants to claim otherwise has the burden of proof. Until they satisfy their burden of proof, I won't say "I don't know", I'll say, "I don't believe you."
To quote Christopher Hitchens, "That which can be asserted without evidence, can be dismissed without evidence." I call myself an atheist not because I know a god doesn't exist but because I have a sense of dignity. I don't have to let church leaders lie to me and demand money for the privilege of listening to them.
Technically, I'm both an agnostic and an atheist. The two aren't mutually exclusive. It would be too charitable (and in my opinion, cowardly) to theists to call myself simply agnostic.
It will give you some insight into where your reasoning has been led astray.
Ryan, I am also an atheist and an agnostic. Reg is what's called a "strong" atheist, which means that he goes so far as to say that there are no gods. I am what is sometimes called a "weak" atheist, which means that although I don't personally believe in a god, I don't claim there are none. Saying I am a weak atheist does not imply that I am on any kind of fence. I am solidly atheistic. Atheism and agnosticism are not necessarily on a linear continuum - there are too many variables.
I am an agnostic because I lack knowledge of the existence of a god. Some agnostics think that nobody can know about a god, but who am I to say what other people can know or not know? I don't think anybody knows about whatever god may or may not exist. I think people make gods up because they need to, and then humanity complicates it, screws it up, and calls it religion.
There was a time when I did not understand this, and it became clearer as I learned more and talked to others. I applaud your efforts!
I would say that I am atheist with respect to all the gods that humans have conjured up so far, but agnostic about the gods of the future. I just need evidence.
You are right about the most honest answer being that we don't KNOW. But we are still human so things we feel certain about are given an elevated status.
As for what there is after death: we have not found any evidence for a spirit / soul / whatever so that concept gets dismissed.
My personal view involves the consideration that we can experience many things in our lives that cause a change in us: love, someone else's death, illness, injury, losing sight or hearing, (re)gaining sight or hearing, etc. Some of those things can strengthen us while others can cripple us. Even just natural aging changes a person. Once you consider all those possible sources of change add them up. That sum is still going to be less than the change caused by death. So if there is something of us that continues on past death, that something is not the person you are before death. So this is very much the only life we have to live - even if something continues on.
So, Heaven and Hell - nope, don't exist. No reason for them to exist since none of us are going there. Whatever continues on (if something does) is alien to us now - even though it used to be us.