Whoever has not realized it yet, here as info before the indexer is publically available:
request_terminate_timeout = 900s
php_admin_value[memory_limit] = 2G
have been set as the default values for 2021.08.2+ - This is fairly enough to process quite large mailboxes. If you should still have a (very, very) large mailbox, you can extend that limit for the initial indexing, triggered by a single search. after that, searches are incremental and are much, much faster.
the indexer will provide some indexing data that can be controlled without user interaction.