Update: 

JS + Google Organic + Wordpress doesn't really apply for most people in this group I would bet but if you have content writers who put out a lot of posts and you are working on SEO as well as paid, maybe. As one option for content I took a look at the WP REST api by getting posts via Postman, and it might work as a content writing environment as it does provide a nice feature of being able to zip up and down hundreds of posts' content with one scroll, but the characters are all encoded, as they have to be, and the posts it gets are messy because I have so much custom HTML in them from columns that Gutenberg couldn't make at the time. Yes there is Elementor and other page builders but I didn't want my post design structure locked in to any of them.

So w/out the REST API, filling in content via logic-less templates is still the best option. The helpful information I can provide here is that I tested a live URL with the JS templating in search console and the screenshot showed the content. So it was client-side rendered JS, but Google still read it. I remember reading, as I mentioned above, that the first index looks at the source, and if the page is not content rich, it takes longer to be fully indexed, and unfortunately I don't know if that's the case in 2019 for normal crawling, just that this live test showed content. The delay on full crawl doesn't really matter to me, but someone with a viral site or content site with daily posts would definitely care. In any case I'll see how Google treats dynamic insertion on pages that it discovers by crawling organically, and for my particular situation with the lingering concern being an invalid JSON file, the site will be kept up for users by maintaining one production, valid JSON file and a separate staging JSON file for writing posts and overwriting the production file. 

I know writing posts in the editor makes the most sense, but it saves time being able to pull bits and pieces from other posts for a new post and being able to correct errors across posts/your brand in one file when you have a lot of posts. Cheers and send me a message if you have any comments/questions about JS + SEO or Wordpress bulk editing. If anybody knows how to do server-side rendering with WP, so that Google definitely sees the content on the first crawl (and so it's a little faster for users), I'd be interested as well.