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, because return values are processed without being stored in a database. There's also no limit for custom low-level discovery rules, however, if it is intended to acquire custom LLD data using a user parameter, then the user parameter return value limit applies (512 KB).
If data has to go through Zabbix proxy it has to store this data in database so database limits apply.