Configure translog parameters and flush period

hey there, I'm trying to understand correctly how to configure translog parameters.
Taking a look to official documentation I have a doubt regarding index.translog.sync_interval
I am moving in this scenario:
index.translog.flush_threshold_size: default to 512mb
index.translog.flush_threshold_ops: default to unlimited so ignored
index.translog.flush_threshold_period: 10m
index.translog.interval: default to 5s

So, translog will be checked every 5 seconds to see if a flush is needed, the flush operation should be performed only if the size value is reached or if the period is expired. is that correct?

then, how should I use index.translog.sync_interval? Is this value used to synchronize in-memory buffer docs and translog?

furthermore, if I check the thread_pool count with _cat/thread_pool?v&h=host,flush.completed I can see the values increasing very quickly (from 10 to hundreds every 2 seconds) but when I check the translog with _stats/translog, the value of size_in_bytes is not 0.

what am I missing?