To specify routes to start up last, you can set up the startupOrder number to much bigger than 1000 (e.g. 10000) and it is the officially supported way.
However, the old doc is still obscure in that whether you can use the numbers >= 1000 for the startupOrder attribute. See:
You can also configure the order in which routes are started. Previously Camel started the routes in a non deterministic order. Now you have fine grained control in which order the routes should be started. There is a new attribute startupOrder which is a Integer that states the order. Camel then sorts the routes before starting time. The routes with the lowest startupOrder is started first. All startupOrder defined must be unique among all routes in your CamelContext.
You should also use numbers that are lower than 1000, as routes without an explicit startupOrder define will have a number starting from 1000 auto assigned. So view numbers from 1000 upwards as reserved internally for Camel itself.
Let's update the doc to clarify this supported way.