Not sure I follow. I've been running public "forwarding" email servers for 20 years, and ... this is exactly what they do. This is exactly what Cloudflare's "forwarding" email solution does (except it has ARC and everyone who wants to support forwarding includes Cloudflare's MTAs in their ARC configuration).

Little guys like me (of course) don't get included in anyone's ARC configuration, to patch over SPF's profound design failure.

If you're describing a "forwarding" solution that doesn't simply do SMTP "RCPT TO" forwarding, then I don't understand what you're describing...