USB4 should be ok. The cable says it’s rated at 40Gbps so it’s supposed to have enough bandwidth to carry a full DP video signal.
The 105 release is “official”, but just hasn’t been make available on the main website for some reason. It appears to be as stable as the 104 firmware version and fixes several bugs. The only potential new bug is the standby coil whine, which only appears to be affecting some people.
I wouldn’t worry too much about the firmware upgrade process. It’s pretty robust, and won’t use the new firmware until it’s been fully received and verified.
I had some issues upgrading to firmware 105. It was flaking out about 2/3 of the way through the upload and restarting. I killed off the firmware updater multiple times at various points during the upload. I power cycled my Spectrum a couple of times. It just kept working using the current firmware.
The only thing that would be a concern is a power cut, if it happens when the Spectrum is applying the firmware you’ve uploaded. Unless there’s warnings about power shortages where you live, just wait for a day when there’s good weather and do the update then.