Mac Admin books?

Take a look at books by Armin Briegel in Apple Books and his blog at https://www.scriptingosx.com. Lots of good information there. 

Others have suggested good resources. You mentioned wanting to learn Apple Script, if you goal is the Jamf 300 stick to bash as Apple Script was not on it when I took it a few years ago. For me, experience is the best teacher, dive in to deploying scripts and extension attributes to solve problems your environment is currently facing. Once you get comfortable with those, you should be in a good place for the Jamf 300.

There are a ton Bash Scripting on youtube and other places. As mentioned above scriptingosx.com is the best.  There is the JAMF Catalog as well.  I also have seen a ton on JAMF's Youtbue channel from previous JNUC's.  

As people have said it's a lot about doing and deploying.  The 300 also doesn't have a lot of scripting, more tweaking a script.  The 400 is all scripting.  Focus a lot on more on advanced deployment, Configuration profiles and API. 

I think from what you have all said it would be more beneficial to learn more about Bash scripting. I'd learned PowerShell in my current role and PowerShell in month of lunches was a good one which got me interested in scripting (and thought potentially AppleScript). While the 300 doesn't involve much scripting I think it would be great to start learning some of that now as 400 has a lot. 

I've got some direction on what to study/look into. I don't have full access on training environment setup but expect might get this soon with a test Mac setup so I can test things and have a play about and learn/break + fix things.

I've some ideas but is there any really good sites or suggestions on what type of policies, configurations, scripts, extension attributes etc to try setting up and testing? What kind of things have you scripted/setup/broke/fixed? Or what are likely real-world scenarios that would need configuration in JAMF to resolve/remediate? I'm just looking for fresh ideas on what to configure. I know I can do the basic stuff like packaging an app like Chrome then doing a post install script to run an update inventory or update favourites etc... but I'm looking for ideas on more "meaty type" things to configure/learn more about. 

You're going to need to dive deep into the file systems and their functions. I have over 24 years of experience on macOS (and through all the name changes). When I was teaching Windows admins, the first thing I did was go through the file system—that is, the Library, File System Library, and the UNIX-based hidden directories. Understand what the preferences files were doing and hierarchy. 

Also, all these configurations that JAMF pushes out are listed on the Apple Developer site. Knowing how to create a custom configuration profile from scratch is a good tool, and I constantly do this. Find a configuration profile and use command defaults to change its setting. Relaunch that service and see how that changed on your system. 

https://developer.apple.com/documentation/devicemanagement

This book helped me.

Learning Unix for Mac OS X

https://www.amazon.com/Learning-Unix-Mac-OS-2nd/dp/0596004702