There is nothing built into Sidekiq to complete this, but obviously, you can just use Redis commands to do it.

Here is code snippet to do that.

from_queue_name = 'default'
to_queue_name = 'low'
count_block = proc { Sidekiq.redis do |conn|
  conn.llen("queue:#{from_queue_name}")
end }

while count_block.call > 0
  Sidekiq.redis do |conn|
    conn.rpoplpush "queue:#{from_queue_name}", "queue:#{to_queue_name}"
  end
end

This code snippet will move all the items from one queue to another until there are no more jobs.

With a full engine, the parent application inherits the routes from the engine. It is not necessary to specify anything in parent_app/config/routes.rb. Specifying the gem in Gemfile is enough for the parent app to inherit the models, routes etc. The engine routes are specified as:

# new_engine/config/routes.rb 
Rails.application.routes.draw do 
  # whatever 
end

No namespacing of models, controllers, etc. These are immediately accessible to the parent application.

I added routes directory to application config directory and added it in autoload_paths to ensure that routes are reloaded when these files change.

# config/application.rb
config.autoload_paths << Rails.root.join('config/routes')

Then moved necessary routes to small files like this.

To clear the queue of jobs, I ran the following commands as listed in Sidekiq wiki.

Sidekiq::ScheduledSet.new.clear
Sidekiq::RetrySet.new.clear

Sometimes, these commands would clear the queue. Other times, you would need to clear the queue from Redis itself.