Hi @mbezzo I like to use the built in defaults command to build custom plists for creating custom profiles.
Description from the man page: Defaults allows users to read, write, and delete Mac OS X user defaults from a command-line shell. Mac OS X applications and other programs use the defaults system to record user preferences and other information that must be maintained when the applications aren't running (such as default font for new documents, or the position of an Info panel). Much of this information is accessible through an application's Preferences panel, but some of it isn't, such as the position of the Info panel. You can access this information with defaults

I also like to use an application named PlistEdit Pro for modifying or just viewing certain keys. More information can be found here https://www.fatcatsoftware.com/plisteditpro/

Not many people mention it, but a tool built a while back (5+ years now) from Tim Sutton still works nicely to take a crafted plist file and convert it into a .mobileconfig profile. It's called mcxtoProfile. I've used it a number of times and it does a good job. The only thing is, I usually make a few small manual edits to the config file after in a text editor since it tends to create Payload Identifiers with a string that looks something like this MCXToProfile.7eeb4421-f945-4633-b959-a96f3915adde.alacarte.customsettings.0a67188d-8dfa-4638-b279-c520042796c1 I edit those to make them more human readable and standardized. But that's just me and my neurosis. :) It does not affect anything to leave them as is.

I mostly just upload a .plist into a Custom Settings Payload.

I haven't found any benefit to building the full .mobileconfig file.

@mbezzo I used to create them manually in a text editor, but there's a relatively new tool called ProfileCreator that's like pressing the "easy" button :)

https://github.com/erikberglund/ProfileCreator/releases

Thanks everyone! Appreciate all the suggestions. Will be checking them out!