My criteria include, but are not limited to:
Unfortunately, I think criteria 2 & 3 eliminate the entire field, or at least the ones that have a real chance to be the nominee.
I’m not aware of any that meet your criteria.
I’d be as happy as that proverbial pig in shit if there were such a candidate.
Are these issues really more important to you than all other issues combined? You’d rather lose health care, see us get into a war, and face a recession than register your gun with the government?
Did you skip #1?
Not sure what you mean by “lose health care”. I’m not a fan of the “medicare for all” plans I’ve seen mentioned. We’re already in a war, and have been for years. And all of my guns were lost in a tragic boating accident in January, 2009.
So on point number one, we have several Dem candidates whose policies are generally mainstream (Biden, Buttigieg at the top of the list). We have one Republican candidate whose policies are far outside the mainstream.
I’m not sure why the Democratic candidate has the burden of proof here. (Disregarding the other points for the moment.)
Wow, I wish I had done the same, since Obama came to my house straight from the inauguration and confiscated all my guns. Oh, wait, that’s not what happened at all.
It was a bad time for a lot of (former) gun owners. 
I’m looking for a candidate to support. Trump is a complete non-starter for me. Didn’t vote for him last time, won’t vote for him this time. I did have an opportunity to watch one of his rallies on television a few weeks ago. Sounded like he’d fit right in had he spoken in Berlin in 1938.
If I can’t find a suitable Dem, I’ll cast my ballot for a third party, likely Libertarian, or write myself in. Hence, this thread.
Ok, sounds like you know what you’re looking to do already.
AFAIK, all Dem candidates are in favor of banning sales of 'assault weapons". If you already have one, that shouldnt affect you. And Trump was in favor of that also.
However, remember unless the Dems win the Senate (could happen), and have a solid majority in both houses (wont happen), no real gun control legislation will be passed. And the candidates know that, so they can posture a lot.
I would caution you not to let perfect be the enemy of the good.
This is more of a case of not letting the turd be the enemy of the dungheap. All the choices are shit. I didn’t vote for Trump last time and will not this time. He’s a buffoon and I’ve disliked him since the 1980s. However, none of the possible Democratic candidates are people I would vote for, either. I’ve disliked Joe “Peckerhead” Biden since his first run at the Oval Office many years ago.
I too will vote 3rd party if the Democrats nominate any of the present likely candidates.
Oh man!
And this is what got us four years of trump.
Define “moderate.” Because from what I’ve seen from other posts on this board, “moderate” has a pretty wide range.
I’d say look beyond the platform and decide what you think they’d actually do in office. If McCain had won in 2008, does anyone really think he would have gone batshit on social issues? I doubt he would have made abortion and banning gay marriage over and over a focus of his presidency. It would have been all foreign policy along with dealing with the financial crisis.
As far as guns, although I don’t know Indiana’s deer season laws, I have lived in the Midwest a good chunk of my life and deer hunting is a big thing here. Pete Buttigieg, as a midwestern mayor, is hardly likely to make guns a focus of his presidency.
“All the choices are shit”
Maybe the problem is your expectations. There doesn’t seem to be much of a political push from any direction for a candidate that would fit all of your expectations. This is politics. There will never exist a perfect candidate just like there will never exist a perfect person. At some point though, we have to make a choice, and voting 3rd party in many cases in 2016 was a vote for Trump.
The “Ooga Booga! Democrats are going to take away your guns!” meme has been sold to the public so effectively that IMHO there is nothing, and I mean ABSOLUTELY NOTHING, that a D candidate could do or say that would convince people otherwise. So, that’s kinda baked into any of these specious discussions, innit?