Need suggestions on how to figure out why a remote household is seeing random buffering issues

Did you already have a look at the related troubleshooting support article?

https://support.plex.tv/articles/200931138-troubleshooting-remote-access/

There’s also a guide with some more details. That being said… if there issue only happens in for a specific user/location, the issue might not be with your server/ISP but their network (maybe their ISP filtering/limiting traffic to/from certain ports). In anyway it might be useful to check those steps to rule out there’s a more generic issue.