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I also found this, which may be helpful: https://github.com/allan23/ep-troubleshoot - doesn't seem to be in the plugin marketplace so you'd have to install it manually, but may be helpful in generating more debug info specific to what's happening in elasticpress land.

Looking more closely at the exception being logged during indexing:

Caused by: SearchParseException[failed to parse search source [{"from":0,"size":5,"sort":[{"post__in":{"order":"desc"}}],"query":{"match_all":{"boost":1}},"post_filter":{"bool":{"must":[{"bool"
:{"must_not":[{"terms":{"terms.product_visibility.name.raw":["outofstock"]}}]}},{"bool":{"must":{"terms":{"post_id":[56662,57698,58086,10463,32389,63478,60303,60287,60277,60308,60268,15626,65731,65444,65712]}}}},{"term":{"post_type.raw":"product"}},{"term":{"post_status":"publish"}}]}}}]]; nested: SearchParseException[No mapping found for [post__in] in order to sort on];
I would guess the issue is with one of those post ids: 56662,57698,58086,10463,32389,63478,60303,60287,60277,60308,60268,15626,65731,65444,65712

I've confirmed there are no port disconnect or ES resource issues occurring. I even bumped the memory for ES up to 2GB from the standard 1GB.

Have you examined the data pertaining to those post ids?