How often is "too often" for emailing journal editors? Context: I submitted a paper for initial review May 2018, and not long after received a "revise and resubmit" response. I never got a confirmation regarding my resubmission, and the email I sent requesting that wasn't answered. A few months later, I checked in again, and finally was told they never received it. I resubmitted it (it's now January 2019, by the way.) Since then, I have reached out a few times to check in. Once in May - the associate editor said they were still waiting on the evaluations. I wrote again in August, only because I saw that the associate editor had changed, and I correctly assumed that the new one didn't know anything about my submission. When they did look into it, they said it was still in review. Against my better judgement, I checked in again in October (I didn't want to be annoying, but...in my defense, my previous submission had been lost, and this most recent submission had been sent way back in January.) However, I got no response. And here we are in December, shortly approaching the 1 year mark, and still no update.

Finally the question: would it be inappropriate to send another email some time soon? On the one hand, I completely empathize that people are busy and volunteering their time when serving on an editorial board, and I don't want to annoy them over things that are maybe out of their control. However, they don't have the best track record (I also consulted with another colleague who similarly had a frustrating experience with this journal) and keeping my paper in review for almost a year seems excessive (in my field the turnaround for a review is usually about 4 months). If sending another email is in order, any suggestions on how to inquire and point out the delays and unresponsiveness without sounding annoying or like a jerk?
As you point out, you've waited close to a year. You shouldn't feel bad about asking for updates after this long, especially since it's been two months since you last checked.

In your email I'd point out that it's been almost a year since you resubmitted, and you're wondering what exactly is happening. When can you expect a decision? If they can't predict that, then when are the reviews due?

If the editorial board member handling your paper is unresponsive, you could try asking the journal office instead, although it sounds unlikely the journal office will be able to say much (because my guess is the reviewers never agreed to review and/or never actually submitted a review, so the only way forward is to invite more reviewers, but the handling editor hasn't done it).