Hi Anthony,
Thank you for posting your question in this community. It seems that you have some concerns about SharePoint usage based on your description. Hence, I would like to share my knowledge and experiences based on your question separately. Hope it can be much clearer.
1. Is it common practice to maintain document content in a team site, but publish it (via link) to a communication site?
I am not quite sure what is the final object you would like to achieve. But you can list the document from a Team site to another community site by creating a sharable link of the document as long as the sharable link is not expired and the users in communication site have the related permission to access this document.
For the question 2 and 3, it seems that they related to each other, so I put them together.
2. Are content types better for identifying document types (policy, guide etc.) than terms?
3: Are content types the only method of associating O365 data polices?
Generally, both content types and terms in term set are commonly used for managing and indexing your data storage in an easily way.
For content types, it provides an ability to organize the contents in SharePoint in a centralized and meaningful manner. For example, in a SharePoint site, you have varieties of files for different projects. By creating a content type, like project 1, project 2. Then, you can assign the content type according to different files based on different projects. In another words, you can figure out which files belong to project 1, which files belong to project 2
For your information:
For the terms in term set, you can think it as a label or category in library. For example, a SharePoint site contains a lot of information and contents which are belongs to different company departments. Then you can apply this “label” according to different departments, such as HR, Accounting, Marketing, etc.
For your information:
Overall, the purpose of these two are helping the site owner or users to manage and search the related information and contents in SharePoint in a much efficient way.
For the Microsoft 365 policy you are referring to, may I know it is the policies, such as Data lost Policy (DLP), Retention policy etc.? If so, based on my knowledge, the content types and terms didn’t determine by these policies. For example, Data lost Policy in Microsoft 365, which it helps to protect sensitive information in your organization tenant, such as customer credit card information, social security numbers, etc. After you set up the rules based on your requirement. It would scan all the locations you applied and figure out whether there is any sensitive information in these locations.
For your information
Overview of data loss prevention
Overview of retention policies
4. How can I safely define a content type in a central location that is available to existing team sites? - Some users have suggested not altering the default content type hub, though what is the workaround?
Generally, each site, such as site collection, site library, site list has its own content types. You can create or manage the content types from highest level to lowest level sites and the content types in lowest level can also inherit the content type from higher level, For example, a site document library can inherit content type from the site level.
I am not quite sure which is the “safely define a content type in a central location. you can also share more details of it with a objective you may would like to achieve.
5. Are general information (communication pages) best created in the Team site, or a Communication site, when the intent is for all internal users to access them (Who we are, what we do in our department etc.)?
In this scenario, Communication site would be more suitable compare to Teams site as you tend share the information across all the internal users in your tenant from my point of view.
For your information:
Communication Site vs Team Site
6. How can a document be locked from further editing? - I understand that 'approval' does part of this.
In most of the senarios, to prevent document for further editing by users, you can always use check in or check out the documents for “approving”
Check out, check in, or discard changes to files in a library
However, you can also consider use to the SharePoint workflows for approval as another option to achieve this object.
Best regards,
Dihao