Hi @Rutger, I had exactly the same problem, the same roads every time or intervals ending up in a descent 
so I built it: Gritmaps: routes for your intervals and it’s available on https://gritmaps.com
It takes the planned workout from your Intervals.icu calendar and searches loops from your door where the terrain fits the efforts: climbs and headwinds for the hard blocks, descents and tailwind for the recoveries, and it penalises efforts broken up by junctions, traffic lights or urbanities.
For the triggering issue you mention with Garmin, it pushes the workout with each step ending at a distance on the route, so the step changes where the climb starts without pressing lap.
Free to try, curious to hear if it matches what you had in mind!