I find that the high G is sometimes as much as a semitone sharp. One solution would be to try to use less air pressure when playing the note, but that's tricky. Currently I am using electrical tape to partially cover the G hole, but that has the problem of inconvenience and also crushes the sound of the G grace note. Is there a better solution?
There are some excellent suggestions at https://appsreeds.com/pages/dealing-with-a-sharp-high-g (Some of which you’ve tried already).
The other thing to say is that practice chanters aren’t always made to the most exacting of standards, so it may be something that can’t be fixed like an incorrect hole placement, a resonating chamber that has not been precision engineered, or a poor quality reed (although the reed can be swapped out of course).
If it’s a new chanter sold to you from a reputable instrument maker you should contact them and discuss the issue direct - if it’s by an established maker they may even be prepared to help if it’s one of theirs but you bought it second hand. If it’s a no-name practice chanter bought off ebay or from a shop where the chanters were displayed next to the tartan scarves and the shortbread, it’s probably not going to be of that quality.
If this is helpful, the tape should only be used to change the pitch of certain notes, while making it easier to fit your fingers. I would suggest getting a new chanter and starting afresh. Maybe visit the manufacturer and question about this as it might not be your fault. That's all I really know in how to fix the problem. I have only had experience with bagpipes practice chanter, but it has never failed me...