You can do this with the sticker spray brushes i.e. have pressure affect opacity but not with the real media tools.
As AR aims to replicate real world paint this is possibly a good thing.
In the real world brush or pencil pressure may well effect line width and/or the amount of media deposited onto a surface but it won't affect the actual opacity of said media. This is a inherent property of the media which can not be altered by the tool applying it.
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