Not sure if Hetzner has IPv6 but try binding your mount to your IPv4 address, Google sometimes limits peering with IPv6 and you end up doing a longer route, pretty sure Hetzner should have direct peering with Google so your ping time looks high in double digits.
Does your current ISP offer a speedtest.net server in your area? If so, try running a speedtest from your VPS server to your ISP server in your area, this will tell you the upload/download speed of your ISP and Hetzner in your area, it might be the downlink from your ISP is fast, but the uplink slow/congested, since it's the other way around you are getting limited.
Finally, if you have a computer to test, try mounting it in your home and do a test there. See if it buffers or not. For 4K the less trip your stream takes the better.