Yes sorry that was a little confusing. As far as I know, even though they are the same principal, according to the book a taxi or departure report does not require a pre-emption, but a STAR clearance does.
I really don't mind if you do or don't pre-empt a taxi or departure report, but it obviously annoys some others if you do.
Fortunately I've never worked HF so I'm not that familiar with the protocols for calling. However Flightwatch won't know you exist until you call them. The ATC on VHF is the one holding your sarwatch, HF can be considered a third party relay, much as if you ring briefing to cancel by phone. They take your message and pass it to us.
In two way VHF coverage it shouldn't risk over transmitting anymore than any other transmission. The original intent was to give ATC time to prepare before you went ahead. These days though the system is better and we are familiar with it.
Now a taxi call only requires me to find your strip in the preactive strip window and mouse click on it once. This brings up a track and associated label data block (callsign, requested level, track, a/c type etc) at the departure point, and automatically sets a timer for now+10 minutes for sarwatch. Another few clicks and I can record any traffic passed.
Previously I would have to process your electronic strip on the screen, and then write your callsign, taxi time, sarwatch time and any traffic on a sheet of paper, while still scanning the screen and the sheet of paper for traffic, expired sarwatch times etc.
Bear in mind we had gone from a paper strip system that we had years if not decades of experience with to something very different and frankly, at the time, not very good. Everything that we used to do instinctively and automatically was new and complicated. The pre-emption was a risk mitigator (amongst others).