There are several services that can help you to manage your websites like YourSites, MySitesGuru, BackupMonkey... to mention a few...

Are you aware that there is a range of commands available at the command line that can help you?

php cli/joomla.php

Usage:
  command [options] [arguments]

Options:
      --live-site[=LIVE-SITE]  The URL to your site, e.g. https://www.example.com
  -h, --help                   Display the help information
  -q, --quiet                  Flag indicating that all output should be silenced
  -V, --version                Displays the application version
      --ansi                   Force ANSI output
      --no-ansi                Disable ANSI output
  -n, --no-interaction         Flag to disable interacting with the user
  -v|vv|vvv, --verbose         Increase the verbosity of messages: 1 for normal output, 2 for more verbose output and 3 for debug

Available commands:
  help                            Show the help for a command
  list                            List the application's available commands
 cache
  cache:clean                     Clean cache entries
 config
  config:get                      Display the current value of a configuration option
  config:set                      Set a value for a configuration option
 core
  core:check-updates              Check for Joomla updates
  core:update                     Update Joomla
 database
  database:export                 Export the database
  database:import                 Import the database
 extension
  extension:discover              Discover extensions
  extension:discover:install      Install discovered extensions
  extension:discover:list         List discovered extensions
  extension:install               Install an extension from a URL or from a path
  extension:list                  List installed extensions
  extension:remove                Remove an extension
 finder
  finder:index                    Purges and rebuild the index
 scheduler
  scheduler:list                  List all scheduled tasks
  scheduler:run                   Run one or more scheduled tasks
  scheduler:state                 Enable, disable or trash a scheduled task
 session
  session:gc                      Perform session garbage collection
  session:metadata:gc             Perform session metadata garbage collection
 site
  site:down                       Put the site into offline mode
  site:up                         Put the site into online mode
 update
  update:extensions:check         Check for pending extension updates
  update:joomla:remove-old-files  Remove old system files
 user
  user:add                        Add a user
  user:addtogroup                 Add a user to a group
  user:delete                     Delete a user
  user:list                       List all users
  user:removefromgroup            Remove a user from a group
  user:reset-password             Change a user's password

Unfortunately you can't update an extension via CLI. You can only get the information that there are updateable extensions, not even which ones, let alone updating them.

please give a look at #42829

"I aim to develop a custom administration tool to manage multiple websites hosted on various ISPs. Utilizing a Command Line Interface (CLI) won't suffice as it necessitates internal system handling."

There are many options for this already available including Panopticon which is completely free

There are many options for this already available including Panopticon which is completely free

Thanks!!!