ok, enough of running a detuned database to reproduce problems. Just configured PostgreSQL 15 with:

# DB Version: 15
# OS Type: linux
# DB Type: web
# Total Memory (RAM): 24 GB
# CPUs num: 4
# Connections num: 500
# Data Storage: hdd

max_connections = 500
shared_buffers = 6GB
effective_cache_size = 18GB
maintenance_work_mem = 1536MB
checkpoint_completion_target = 0.9
wal_buffers = 16MB
default_statistics_target = 100
random_page_cost = 4
effective_io_concurrency = 2
work_mem = 6291kB
min_wal_size = 1GB
max_wal_size = 4GB
max_worker_processes = 4
max_parallel_workers_per_gather = 2
max_parallel_workers = 4
max_parallel_maintenance_workers = 2