Query take a long time and use no index

Hello,

I use volkszaehler.org (a DIY Smartmeter) on postgres.

The hardware is a Dual-core 2,4 GHz amd64-CPU and 6 GB RAM.
The databse is approx. 1.5 GB on SSD

my local config looks like:

root@vz:~# cat /etc/postgresql/15/main/conf.d/local.conf
# DB Version: 15
# OS Type: linux
# Total Memory (RAM): 6 GB
# CPUs num: 2
# Connections num: 20
# Data Storage: sdd

max_connections = 20
shared_buffers = 2GB
effective_cache_size = 4608MB
maintenance_work_mem = 384MB
checkpoint_completion_target = 0.9
wal_buffers = 16MB
default_statistics_target = 100
random_page_cost = 0.9
effective_io_concurrency = 200
min_wal_size = 1GB
max_wal_size = 4GB
work_mem = 512MB
temp_buffers = 512MB

# log
log_directory = '/var/log/postgresql/'  # directory where log files are 
written,
                                         # can be absolute or relative 
to PGDATA
log_filename = 'postgresql-15-main.log'
#log_statement = 'all'
#logging_collector = on
log_temp_files = 0
session_preload_libraries = 'auto_explain'
auto_explain.log_min_duration = '3s'


The Values:

volkszaehler=# SELECT *  FROM data limit 10;
    timestamp   | channel_id | value
---------------+------------+-------
  1666008690865 |          5 |   232
  1666008691867 |          5 |   236
  1666008692865 |          5 |   237
  1666008693867 |          5 |   232
  1666008694865 |          5 |   227
  1666008695867 |          5 |   229
  1666008698866 |          5 |   228
  1666008699868 |          5 |   229
  1666008700864 |          5 |   228
  1666008701868 |          5 |   230


volkszaehler=# SELECT COUNT(value) FROM data;
   count
----------
  17884247
(1 row)


The Indexes:

volkszaehler=# SELECT tablename,indexname,indexdef FROM pg_indexes WHERE 
tablename LIKE 'data%' ORDER BY tablename,indexname;

  tablename |           indexname            | 
                   indexdef

-----------+--------------------------------+--------------------------------------------------------------------------------------------------
  data      | idx_16391_idx_adf3f36372f5a1aa | CREATE INDEX 
idx_16391_idx_adf3f36372f5a1aa ON volkszaehler.data USING btree (channel_id)
  data      | idx_16391_primary              | CREATE UNIQUE INDEX 
idx_16391_primary ON volkszaehler.data USING btree (channel_id, "timestamp")
  data      | idx_data_timestamp             | CREATE INDEX 
idx_data_timestamp ON volkszaehler.data USING btree ("timestamp")
(3 rows)


The Query:

volkszaehler=# explain analyze SELECT COUNT(DISTINCT DATE_TRUNC('day', 
TIMESTAMP 'epoch' + timestamp * INTERVAL '1 millisecond')) FROM data 
WHERE channel_id = 5 AND timestamp >= 0;

                                                          QUERY PLAN 


----------------------------------------------------------------------------------------------------------------------------
  Aggregate  (cost=590793.68..590793.69 rows=1 width=8) (actual 
time=15449.536..15449.539 rows=1 loops=1)
    ->  Seq Scan on data  (cost=0.00..382037.82 rows=16700469 width=8) 
(actual time=247.092..3833.495 rows=16669429 loops=1)
          Filter: (("timestamp" >= 0) AND (channel_id = 5))
          Rows Removed by Filter: 1215163
  Planning Time: 0.374 ms
  JIT:
    Functions: 5
    Options: Inlining true, Optimization true, Expressions true, 
Deforming true
    Timing: Generation 1.201 ms, Inlining 158.833 ms, Optimization 
59.816 ms, Emission 28.472 ms, Total 248.322 ms
  Execution Time: 15451.093 ms
(10 rows)

Round about 16 sec is too long, the frontend run in timeout or other erros.

A simple count look like

volkszaehler=# explain analyze SELECT count (channel_id) FROM data;
                                                                  QUERY 
PLAN

---------------------------------------------------------------------------------------------------------------------------------------------
  Finalize Aggregate  (cost=208089.76..208089.77 rows=1 width=8) (actual 
time=3514.293..3523.842 rows=1 loops=1)
    ->  Gather  (cost=208089.55..208089.76 rows=2 width=8) (actual 
time=3514.247..3523.800 rows=3 loops=1)
          Workers Planned: 2
          Workers Launched: 2
          ->  Partial Aggregate  (cost=207089.55..207089.56 rows=1 
width=8) (actual time=3427.139..3427.141 rows=1 loops=3)
                ->  Parallel Seq Scan on data  (cost=0.00..188457.44 
rows=7452844 width=8) (actual time=0.137..2121.695 rows=5962263 loops=3)
  Planning Time: 0.247 ms
  JIT:
    Functions: 11
    Options: Inlining false, Optimization false, Expressions true, 
Deforming true
    Timing: Generation 2.665 ms, Inlining 0.000 ms, Optimization 1.612 
ms, Emission 48.580 ms, Total 52.857 ms
  Execution Time: 3525.656 ms
(12 rows)

Why are the indexes not used ?
Is it a Problem of the DATE_TRUNC ?
How can I optimize?

Best regards