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I previously wrote a post on how to assign more actions to the crown, most notably max brightness mode. However, that app had a hefty price tag if you wanted more than one shortcut.
Button Launcher is a completely free app that allows you to assign two shortcuts to the crown. Unfortunately, max brightness is not one of them but it is useful for launching apps and can still be used for Google Assistant.
If you want to give it a try you can use the link above and install it via the web to your watch or you can try searching it on the watch itself in the Play Store.