Whilst it is not an ideal solution (I have hundreds of routes as well) there are a couple of ways to bring any route to the top of the list in MRA Route Lab and MRA Navigation.
This is my preference.
In the planner find the route you want and zoom in to the full extent on the first waypoint.
Then just move the first waypoint a tiny bit, a millimetre on the map will do it.
This will bring the route to the top in both the Route Lab and in MRA Navigation
Not an ideal solution for some , as it will change the creation date.
I add the info below for those not familiar with MRA Navigation.
Initially, make sure you have an internet connection in MRA Navigation and open the route if you are running it for the first time.
And also make sure you have the relevant map downloaded if you plan to use “Offline” mode in MRA Navigation.
Works on Android and iOS.
You can also add the route to an event, but personally I have never used this option as I have no issue with the creation date being changed.
Instructions for “Events” can be found in the Support pages