LLD macros may be used inside user macro context, for example, in trigger prototypes.
Since Zabbix agent version 3.2 it is possible to define several low-level discovery rules with the same discovery item.
To do that you need to define the Alias agent parameter, allowing to use altered discovery item keys in different discovery rules, for example
There is no limit for low-level discovery rule JSON data if it is received directly by Zabbix server. This is because the return values are processed without being stored in a database.
There is also no limit for custom low-level discovery rules. However, if custom low-level discovery rule data is retrieved using a user parameter, the user parameter return value limit applies.
If data has to go through Zabbix proxy, it has to store this data in the database. In such a case, database limits apply.