File sizes

Try to import the tcx File in https://connect.garmin.com/modern/courses and then from there export to you Device.

But possible the Calculation of TBT will take to long time or crash, so better you disable it. 

I found this in the 1030 Forum:

When you load a course & select ride the Garmin is Calculating the course not Recalculating 

What this means is the Garmin is taking the course file you have loaded and is creating the TBT prompts it will use for navigation. The downside is it also changes the total height gain of the course based on its own internal DEM map. 

The only way to prevent the calculation is to disable turn guidance in the navigation settings  Then you have to set the course you want to use to always display in its settings

You will get the course displayed as a permanent thin line on the map, you won't get TBT and you will have to leave the map displayed so that you can manually follow your course 

Thr Recalculation setting only affects what the Garmin does when you go off-course, disabling it has no effect on the initial calculation stage

There's nothing to stop you loading the course and starting the calculation well before you start your ride just don't select navigate to start. Once it's done you can put the Garmin to sleep & turn it on again when you are ready to go. Once it gets a GPS fix you should get your TBT without interruption 

I would split the 800 km in little peaces from about 50-100 km if you need the TBT and then load  first part, ride it, load second part, ride it .....