WAL accumulating, Logical Replication pg 13

Hi , I'm upgrading a 1.5TB database from postgres 9.3 to postgres 13 on
Debian 10. This is now in an Acceptance stage (DTAP). I have encountered a
problem: the WAL is not being deleted. I now have 1.4 TB of WAL in pg_wal
and my disks are getting full. The oldest WAL file is 18 days old.
I use Logical Replication from the new cluster to another new cluster with
1 subscriber and 1 subscription.

pg_stat_subscription tells me all recent timestamps.
and this:
db=# select * from pg_replication_slots;
-[ RECORD 1 ]-------+-------------
slot_name | my_pub1
plugin | pgoutput
slot_type | logical
datoid | 16401
database | db
temporary | f
active | t
active_pid | 9480
xmin |
catalog_xmin | 269168
restart_lsn | D4/908BC268
confirmed_flush_lsn | E1/25BF5710
wal_status | extended
safe_wal_size |

I've had problems with diskspace on this server, with postgres crashing
because of it, then added more diskspace and postgres recovered. This
doesn't seem to be a problem now.

The *publication* has the options publish = 'insert, update, delete,
truncate', publish_via_partition_root = false
The *subscription* has the options connect = true, enabled = true,
create_slot = false, slot_name = my_pub1, synchronous_commit = 'off'

The log on the publisher says:
2021-05-25 21:25:18.973 CEST [4584] user(at)db LOG: starting logical decoding
for slot "my_pub1"
2021-05-25 21:25:18.973 CEST [4584] user(at)db DETAIL: Streaming transactions
committing after D6/A82B5FE0, reading WAL from D4/908BC268.
2021-05-25 21:25:18.973 CEST [4584] user(at)db LOG: logical decoding found
consistent point at D4/908BC268
2021-05-25 21:25:18.973 CEST [4584] user(at)db DETAIL: There are no running
transactions.
2021-05-25 21:29:49.456 CEST [4614] user(at)db ERROR: replication slot
"my_pub1" is active for PID 4584
2021-05-25 21:29:54.474 CEST [4615] user(at)db ERROR: replication slot
"my_pub1" is active for PID 4584

And on the subscriber:
2021-05-28 21:23:46.702 CEST [40039] LOG: logical replication apply worker
for subscription "my_pub1" has started
2021-05-28 21:23:46.712 CEST [40039] ERROR: could not start WAL streaming:
ERROR: replication slot "my_pub1" is active for PID 730
2021-05-28 21:23:46.714 CEST [19794] LOG: background worker "logical
replication worker" (PID 40039) exited with exit code 1

The postgres settings on the *publisher* are:
max_connections = 100 # (change requires restart)
tcp_keepalives_idle = 120 # TCP_KEEPIDLE, in seconds;
shared_buffers = 50GB # min 128kB
work_mem = 1GB # min 64kB
maintenance_work_mem = 10GB # min 1MB
logical_decoding_work_mem = 5GB # min 64kB
dynamic_shared_memory_type = posix # the default is the first option
max_worker_processes = 20 # (change requires restart)
max_parallel_maintenance_workers = 10 # taken from max_parallel_workers
max_parallel_workers_per_gather = 5 # taken from max_parallel_workers
max_parallel_workers = 15 # maximum number of max_worker_processes that
wal_level = logical # minimal, replica, or logical
max_wal_size = 1GB
min_wal_size = 80MB
#archive_mode = off
max_wal_senders = 10 # max number of walsender processes
wal_sender_timeout = 1min # in milliseconds; 0 disables
max_replication_slots = 7 # max number of replication slots

On postgres settings on the *subscriber*:
max_connections = 100 # (change requires restart)
tcp_keepalives_idle = 120 # TCP_KEEPIDLE, in seconds;
shared_buffers = 25GB # min 128kB
work_mem = 1GB # min 64kB
maintenance_work_mem = 10GB # min 1MB
logical_decoding_work_mem = 5GB # min 64kB
dynamic_shared_memory_type = posix # the default is the first option
max_worker_processes = 20 # (change requires restart)
max_parallel_maintenance_workers = 10 # taken from max_parallel_workers
max_parallel_workers_per_gather = 5 # taken from max_parallel_workers
max_parallel_workers = 15 # maximum number of max_worker_processes that
wal_level = logical # minimal, replica, or logical
max_wal_size = 3GB
min_wal_size = 80MB
#archive_mode = off
wal_receiver_timeout = 1min # time that receiver waits for
max_logical_replication_workers = 10 # taken from max_worker_processes
max_sync_workers_per_subscription = 5 # taken from
max_logical_replication_workers

I've tried increasing wal_sender_timeout and wal_receiver_timeout to 10
minutes each, but this had no positive effect.

Some advice would be helpful
--
Willy-Bas Loos