For example, I want to change this "Grp" button to be unchecked by default. Is this possible? It is really annoying to have to uncheck this every time!
Unfortunately its a bit of a mess since different settings are stored in different places. And while you can use a start-up macro/script to automatically set certain toggles and preferences for you... you might find that half of them get wiped when you load a project that has its own stored settings.
In the case of the Grp option, I have no idea where it is saved. You'd think it would be saved to the Tool as a Tool Property (since that is where the button is located), but that isn't the case. It isn't saved with the UI Config, nor is it stored in the project (although switching projects will reset the button to be on again). At the moment I just keep the button on my custom UI next to my export button so that I at least don't have to go digging for it every time.
As the others mentioned, many settings can be stored via the <Preferences:Store Config>, or <Document:Save as Startup Doc> (for canvas related settings. There is also the "ZStartup Master" plugin, that is part of the ZBrush 2018 installation, which can be found in the Zplugin palette.
Now the Grp button is an interessting one, afaik the setting is stored in the Tool, BUT for every Subtool independent. So different Subtools can have different export settings and by default it is active. Even when dublicating a Subtool that had the option deactivated. I wrote a little script a while ago, that toggles/untoggles the Grp option for all Subtools of the loaded Tool. So while this is not exactly what you were looking for, the attached script might save you some time regardless
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There is also the "ZStartup Master" plugin, that is part of the ZBrush 2018 installation, which can be found in the Zplugin palette.
Now the Grp button is an interessting one, afaik the setting is stored in the Tool, BUT for every Subtool independent. So different Subtools can have different export settings and by default it is active. Even when dublicating a Subtool that had the option deactivated.
I wrote a little script a while ago, that toggles/untoggles the Grp option for all Subtools of the loaded Tool.
So while this is not exactly what you were looking for, the attached script might save you some time regardless