MariaDB 10.1 using too much RAM on Debian 9.12 - mariadb

Question

I am administrating a server with MariaDB and it often uses a lot of RAM - often so much that it crashes because it can't allocate more RAM, and even when it doesn't it can be pretty slow because it swaps. Even when it is running smoothly, I can see with htop that among the processes using the most RAM there are two dozen of /usr/sbin/mysqld processes. Of course I googled it but I barely understand what the settings in my.cnf do, so nothing I changed appeared to have an effect.
The server has:
2 GB of RAM
Debian GNU/Linux 9.12 (stretch)
mysql --version returns: Ver 15.1 Distrib 10.1.44-MariaDB, for debian-linux-gnu (x86_64) using readline 5.2
Here is the content of /etc/mysql/my.cnf:
# This file has been automatically moved from your previous
# /etc/mysql/my.cnf, with just this comment added at the top, to maintain MySQL
# operation using your previously customised configuration.
# To switch to the new packaging configuration for automated management of
# /etc/mysql/my.cnf across multiple variants:
#
# 1. Move your customisations from this file to /etc/mysql/conf.d/ and
# to /etc/mysql/<variant>.conf.d/ as appropriate.
# 2. Run "update-alternatives --remove my.cnf /etc/mysql/my.cnf.migrated"
# 3. Remove the file /etc/mysql/my.cnf.migrated
#
# The MySQL database server configuration file.
#
# You can copy this to one of:
# - "/etc/mysql/my.cnf" to set global options,
# - "~/.my.cnf" to set user-specific options.
#
# One can use all long options that the program supports.
# Run program with --help to get a list of available options and with
# --print-defaults to see which it would actually understand and use.
#
# For explanations see
# http://dev.mysql.com/doc/mysql/en/server-system-variables.html
# This will be passed to all mysql clients
# It has been reported that passwords should be enclosed with ticks/quotes
# escpecially if they contain "#" chars...
# Remember to edit /etc/mysql/debian.cnf when changing the socket location.
[client]
port = 3306
socket = /var/run/mysqld/mysqld.sock
# Here is entries for some specific programs
# The following values assume you have at least 32M ram
# This was formally known as [safe_mysqld]. Both versions are currently parsed.
[mysqld_safe]
socket = /var/run/mysqld/mysqld.sock
nice = 0
[mysqld]
#
# * Basic Settings
user = mysql
pid-file = /var/run/mysqld/mysqld.pid
socket = /var/run/mysqld/mysqld.sock
port = 3306
basedir = /usr
datadir = /var/lib/mysql
tmpdir = /tmp
lc-messages-dir = /usr/share/mysql
skip-external-locking
#
# Instead of skip-networking the default is now to listen only on
# localhost which is more compatible and is not less secure.
#bind-address = 127.0.0.1
bind-address = ***.***.***.***
#
# * Fine Tuning
#
key_buffer = 8M
max_allowed_packet = 16M
thread_stack = 192K
thread_cache_size = 8
# This replaces the startup script and checks MyISAM tables if needed
# the first time they are touched
myisam-recover = BACKUP
#max_connections = 100
#table_cache = 64
#thread_concurrency = 10
#
# * Query Cache Configuration
#
query_cache_limit = 1M
query_cache_size = 8M
#
# * Logging and Replication
#
# Both location gets rotated by the cronjob.
# Be aware that this log type is a performance killer.
# As of 5.1 you can enable the log at runtime!
#general_log_file = /var/log/mysql/mysql.log
#general_log = 1
#
# Error log - should be very few entries.
#
log_error = /var/log/mysql/error.log
#
# Here you can see queries with especially long duration
slow_query_log_file = /var/log/mysql/mysql-slow.log
# slow_query_log = 1
long_query_time = 1
#log_queries_no_using_indexes
#
# The following can be used as easy to replay backup logs or for replication.
# note: if you are setting up a replication slave, see README.Debian about
# other settings you may need to change.
#server-id = 1
#log_bin = /var/log/mysql/mysql-bin.log
expire_logs_days = 10
max_binlog_size = 100M
#binlog_do_db = include_database_name
#binlog_ignore_db = include_database_name
#
# * InnoDB
#
# InnoDB is enabled by default with a 10MB datafile in /var/lib/mysql/.
# Read the manual for more InnoDB related options. There are many!
#
# * Security Features
#
# Read the manual, too, if you want chroot!
# chroot = /var/lib/mysql/
#
# For generating SSL certificates I recommend the OpenSSL GUI "tinyca".
#
# ssl-ca=/etc/mysql/cacert.pem
# ssl-cert=/etc/mysql/server-cert.pem
# ssl-key=/etc/mysql/server-key.pem
character_set_server=utf8mb4
skip_character_set_client_handshake
skip-name-resolve
tmp_table_size= 32M # 128
max_heap_table_size=128M
wait_timeout=60 #120
max_connections=25 # 40
thread_stack=128K
[mysqldump]
quick
quote-names
max_allowed_packet = 16M
[mysql]
#no-auto-rehash # faster start of mysql but no tab completition
[isamchk]
key_buffer = 16M
#
# * IMPORTANT: Additional settings that can override those from this file!
# The files must end with '.cnf', otherwise they'll be ignored.
#
!includedir /etc/mysql/conf.d/
Is something wrong in my my.cnf? Would updating MariaDB help?