@grahamh, I try my best to know them 
…is perfect for S/PDIF. DD+ over S/PDIF can technically work, but your AVR must be able to handle 192kHz via Toslink for it to work. And it has to be able to handle DD+ itself obviously. You’re saying that your AVR can handle 4K, but not HDR. So, DD+ over S/PDIF might even work, considering the age of your AVR. But consult the AVR’s manual to see which samplerates are supported via Toslink or just test with the Vero 4k. (DTS ES (Discrete)/DTS 96/24/Dolby EX should always work via S/PDIF as they’re extensions of non-HD passthrough formats made for the DVD era, ergo for S/PDIF)
This was actually something we recommended to a user on the forum who had an HDMI AVR that couldn’t handle 4K. He wanted to get the HD audio via HDMI, but also 4K to his TV. Some high-end UHD BD players have dual HDMI-out to do this, but obviously the Vero 4k hasn’t. The user chose a HDFury AVR Key (HDMI audio passthrough LG B6P? - #10 by fragment), but I’m not sure whether it solved it for him. Maybe PM him and ask him about his experience…
If you have a good AVR that you want to keep and that can handle all audio formats that require HDMI, I’d look into such a splitter solution (rather than buying a new one or falling back to S/PDIF). It will probably give you the best of both worlds - 4K HDR and HD audio. Nobody of us has tested such a setup though, so you’d have to pretty much find out yourself how it’s done and whether it’ll work as expected.