Hmm. What do you mean long term/sustainable?

The ROI on SBI shares is about 6 months and, as you've seen, a single share doesn't add much to daily posts. Depending on your goals, you'll want to set a balance between using your SBD to buy SBI shares, powering up, and using ethical voting services.

without spamming or any other sleazy tactic.

For sure. Most of the sleazy stuff (at our level) is buying votes using the major bidbots, and that's not even a really efficient way to grow your rewards anyway. Some vote selling services, like @treeplanter and @minnowbooster are slightly different and not, imho, sleazy at all. Particularly when used in moderation.

My absence for 5 days probably didn't help(value went from over $60 to $52 in that short time.)

Not sure I understand this - your account value fluctuates with SB/SBD value (and is estimated in a pretty janky way anway). It doesn't just go down because you don't post.

them regularly. Anywhere I can learn what return on investment I could expect currently in terms of Steem Dollars?

It can vary, but 2.3 to 2.5x raw returns on the initial value is a good rule of thumb. This usually gives you a good % return on SBD after you account for curation and the 50/50 split (even better when you consider that SP is useful in its own right).

Sidebar: Do you think Steem will go below $2 in value any time soon? I keep going crazy from it spiking

I have no idea and no one can predict that. In the very long term, I see steem rising, but in a volatile market like the crypto world, I don't pay attention to the price.