Hi there.
I would recommend looking at backlink profile in OSE and ahrefs to see the spam score and difference between number in total links (in ahrefs) and number in links-which-matter (in OSE). This will get you an idea of how spammy/bad your backlink profile is. Also do competitor comparison. This will tell you how far behind you are.
As for DA drop - DA is relative number, if it went down, it doesn't mean anything until you compare with competitor DA changes. If theirs went down the same or more - good for you. Read this on this matter - https://moz.com/community/q/da-pa-fluctuations-how-to-interpret-apply-understand-these-ml-based-scores
And also do audit and competitor analysis on content. You might think that your content is super awesome, but still, look at what your ranking competitors have, what keywords they are using, how often they are using them and so on.
Hope this helps.
