Thanks for responding to my enquiry, my Hikvision cameras in service are-
DS-2CD2035FWD-I 3MP Hikvision G1 series fixed Bullet firmware V5.6.6 build 210625
DS-2CD2135FWD-I 3MP fixed Dome firmware V5.6.6 build 210625
DS-2CD2135FWD-I 3MP fixed Dome firmware V5.6.0 build 190507.
PTZ DS-2DE4A425IW-DE PTZ (auto-tracking not configured) firmware V5.6.16 build 200925. Though I am not using the PTZ's auto-tracking I would only update the firmware if I was certain that auto-tracking was retained.
I use the Virtual Host feature on my Hik NVR with Linux Ubuntu Mate desktop browsers, or Blue Moon browser under Wine, or ONVIF Manger under Wine. I also use a VLC playlist that picks up the RTSP streams through Virtual Host.
My BI PC has a 2nd NIC for the cameras' subnet, as you would expect.
I suppose I could feed my Linux PC with the cameras' subnet but I don't want to mess with the IP settings or install a 2nd NIC, even then I'd still have an issue with Hikvision plugins, unless newer firmware is browser plugin free.