The COTS solution for some web browsing is Opera Mini, which may still work? It also uses a proxy to prerender and compress websites, and worked ok at 2G speeds last I used it. It used to work well as a java applet and made the wider internet functional on feature phones. Very solid software.

But as a practical matter, what people rely on phones for are services that are app-based. Good luck completing a Venmo transaction or any amount of banking.