One possible reason why a new custom field is not available inthe data stream is that the Salesforce Integration User, which is used to connect to the CRM source, does not have "Read"permissions on that field2. To fix this issue, the administrator needs to grant the appropriate permissions to the Integration User profile or permission set.

Value suggestion is a feature of Data Cloud that allows you to identify and select text attributes from a picklist of options. You can use value suggestion to standardize values across different data sources and improve data quality. References:https://help.salesforce.com/s/articleView?id=sf.
c360_a_data_cloud_value_suggestion.htm&type=5

Calculated Insights are predefined and calculated metrics that can help you build segments. Some of the benefits of Calculated Insights over Segmentation Operators are:
Calculated Insights are better suited for non-trivial calculations, such as calculating a Net Promoter Score as a percentage, which would require multiple steps and operators in segmentation.
Calculated Insights can query engagement data greater than 2 years, while segmentation operators are limited to 2 years of engagement data.
Calculated Insights are better suited for complex queries over multiple objects, such as joining data from different sources or aggregating data across different levels.

This feature can integrate in real time with Salesforce CRM. Data Actions allow you to perform actions on Salesforce CRM records based on Data Cloud data, such as creating, updating, or deleting records. References:
https://help.salesforce.com/s/articleView?id=sf.c360_a_data_actions.htm&type=5

A data stream cannot be deleted if any of its attributes are mapped to a Data Model object or used in a Calculated Insight3. These dependencies need to be removed before deleting the data stream.