Postgresql 11: terminating walsender process due to replication timeout

I have found some questions about the same error, but didn't find any of
them answering my problem.

The setup is that I have two Postgres11 clusters (A and B) and they are
making use of publication and subscription features to copy data from A to
B.

A (source DB- publication) --------------> B (target DB - subscription)

This works fine, but often (not always) when the data volume being inserted
on a table in node A increases, it gives the following error.

"terminating walsender process due to replication timeout"

The data volume at the moment being entered is about 30K rows per second
continuously for hours through COPY command.

Earlier the wal_sender_timeout was set to 5 sec and I would see this error
much often. I then increased it to 1 min and the frequency of this error
reduced. But I don't want to keep increasing it without understanding what
is causing it. I looked at the code of walsender.c and know the exact lines
where it's coming from.

But I am still not clear which parameter is making the sender assume that
the receiver node is inactive and therefore it should stop the wal_sender.

Can anyone please suggest what changes I should make to remove this error?

sourcedb=# show wal_sender_timeout;
wal_sender_timeout--------------------
1min
(1 row)

sourcedb=# select * from pg_replication_slots;
slot_name | plugin | slot_type | datoid |
database | temporary | active | active_pid | xmin | catalog_xmin |
restart_lsn | confirmed_flush_lsn
------------------------------------+----------+-----------+--------+----------+-----------+--------+------------+------+--------------+----------------+---------------------
sub_target_DB | pgoutput | logical | 16501 |
sourcedb | f | t | 68229 | | 98839088 |
116D0/C36886F8 | 116D0/C3E5D370

targetdb=# show wal_receiver_timeout;
wal_receiver_timeout----------------------
1min
(1 row)

targetdb=# show wal_retrieve_retry_interval ;
wal_retrieve_retry_interval-----------------------------
5s
(1 row)

targetdb=# show wal_receiver_status_interval;
wal_receiver_status_interval------------------------------
2s
(1 row)

targetdb=# select * from pg_stat_subscription;
subid | subname | pid | relid |
received_lsn | last_msg_send_time |
last_msg_receipt_time | latest_end_lsn | l
atest_end_time------------+------------------------------------+-------+-------+----------------+-------------------------------+-------------------------------+----------------+-------------------------------
2378695757 | sub_target_DB | 62371 | |
116D1/2BA8F170 | 2021-08-20 09:05:15.398423+09 | 2021-08-20
09:05:15.398471+09 | 116D1/2BA8F170 | 2021-08-20 09:05:15.398423+09

Increased the wal_sender_timeout to 5 mins and the error started
appearing more frequently instead. Not only that, it even killed the
active subscription and stopped replicating data. Had to restart it.
So clearly, just increasing the wal_sender_timeout hasn't helped.

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