If you really want to engineer web products for users at the edge of the abyss, the most robust experiences are going to be SSR pages that are delivered in a single response with all required assets inlined.

Client-side rendering with piecemeal API calls is definitely not the solution if you are having trouble getting packets from A to B. The more you spread the information across different requests, the more likely you are going to get lose packets, force arbitrary retries and otherwise jank up the UI.

From the perspective of the server, you could install some request timing middleware to detect that a client is in a really bad situation and actually do something about it. Perhaps a compromise could be to have the happy path as a websocketed react experience that falls back to a ultralight, one-shot SSR experience if the session gets flagged as having a bad connection.