Building that aerobic base that Ueli was referring to does take years. I have known of one world champion who saw improvements in his pace ar aerobic threshold for 10 years using the same base building approach of lots of sub AeT training volume before adding too much intensity.
A LCHF diet can help jump start this process but as you can see in your friends who eat HC, fat adaptation is also strong effected by aerobic base training volume. Once you are over probably around 16 hours of aerobic base work in a week then I’ve seen (with many athletes I’ve coached) that diet makes very little difference in fat adaptation.
Be patient and don’t get too caught up with the stuff you mention: cold, near IR, circadian alignment etc. These things might work but none of the top athletes I work with or have ever worked with have used them and some are the their sports highest level. Don’t sweat the small stuff. Put in the volume week after week, month and month and you will see the changes you seek.