Huge SERP drop after HTTPS implementation

You can try use HSTS if you really serve all your content and link in https..
HSTS will help you eliminate redirect, speed up FTB and increase security...
don't know this will effect in SEO or not..

https://searchenginewatch.com/sew/news/2363270/https-doesn-t-boost-rankings-study

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Google recently began recognizing HTTPS as a signal for rankings, but said HTTPS carries much less weight than factors like quality content.

Google also said this new signal would affect less than 1 percent of global search queries, so it’s perhaps not surprising there was not a significant spike in results.

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From personal experience, HTTPS gave me a drop in rankings and continuously building links helped me get back up. But my site can be different from yours because every site has its own structure, on-page, backlinks, etc. so there will be varying experiences. I'm just speaking from my own experience.

I went back to HTTP my rankings did not return after 2-3 months. Been a week and a half and seen some small positive signs back on HTTP. Ill keep you posted as it could be an option?

It's possible that changing your site's HTTP URL into HTTPS would send a signal to Google that the old page HTTP isn't there anymore, and they may take some time to get the HTTPS site indexed instead. It may also temporarily send a signal to Google that your site's pages are temporarily missing, and you will have to wait for their spider to re-index the HTTPS links back into their site. And when they do, you may also need to wait for their search engine to re-categorize and place importance based on the previous signals. The drop in ranking may just be temporary, IMO.