Hey all!
I write just as fyi, i continued to search for a device to use as a splitter for the whole signal (10gbps, 4k hdr hdcp2.2 ecc) and i found these two on amazon.it, which both seems to work (reading reviews and i asked too).
And the last one also has a downscaler built-in, which (it seems) lets pass one full 4khdr signal output and one 1080 output (really useful for the rpi usb grabber).
I haven't tested it nor bought yet, but maybe i will because i tried the external camera grabber and the results aren't enough satisfying (difficult to find a position for the camera, too much equipment in the room and wifi lags, sadly).
Cheers!