As a formerly avid telemarker back in the mid/late-1980s and a runner since, I can say that running never helped me much the few times I ever pulled out the old tele-skis (Karhu Extreme, so we are talking pretty old). Different demands, as you describe from the other direction. So seemingly there is a lot of aerobic base building to be done. However, by your description of what you are doing when you bike, it gives the impression you are working too hard, especially how you describe on a bike trainer. Your approach to training may be subconsciously biased from your long experience from telemarking, as suggested by your references to power output and strength intervals. Endurance building is slow, steady work.
Wiser voices will likely/hopefully come with better comments, but what you describe is essentially one of the key topics Uphill Athlete focuses on – aerobic deficiency: https://uphillathlete.com/aerobic-deficiency-syndrome/