How to preserve muscles on 6hr+ endurance days (Bike Tours/Hikes/Raves)?

Pretty much yes. And listen to your body in how different foods make you feel on the bike. For me headaches and low appetite/mild nausea after a ride were the big indicator I hadn't eaten enough.

Well done you.. meanwhile sport science and the experience of most amateur cyclists I know, including myself, say fancy sugar is actually extremely helpful.

Meaning, just go the naive route and fuel with gels for high intense sessions and muesli bars and fruits for light tours?

Fancy sugar is not going to address lactic acid in muscles for long rides. Internet cyclists love sugar intake, yet somehow I avoided this my entire life.

That works up to a point, but not on multiday rides and definitely not when doing high intensity stuff. And "carb loading" is recommended to be done for 2-3 days, not just the night before.

It depends a lot on how intense your activities are. 6+ hours easy pace where you are not out of breath and can maintain conversations means a lot of energy can still come from fat burning, so you can fuel more easily in a few large wholefood meals with some candybars or snacks in between. The gels and stuff come in when you go more intense for long durations as you use more carbs and when carbs are depleted you start using protein for energy.

I make my own gels with 100 gram maltodextrine, 50 gram fructose and a bit of pectine, in 100 ml liquid (boiling water or water+flavoring like espresso/lemon juice). You can put them in refillable babyfood pouches and store in the fridge for a few days. My gut likes them because they are easy to absorb and don't contain unusual flavorings and chemicals. For 6+ hour efforts I would stop at 60 grams/hour just because the longer duration makes it harder to tolerate high sugar amounts for the gut, but that's personal and can be trained.

The gel recipe came from a youtube video.. if I find it back I'll add the link.

Are you a competition bodybuilder prepping for a comp? If yes, the answer is that you simply should not be engaging in activities like this which might impact your showing.

If your goal, however, is to fuck the festival baddies, then I would simply accept a bit of fat gain and muscle loss during these events. You want to be the hot guy who is having a great time - not the bodybuilder bro who is way too caught up in his macros. The exact definition of your cum gutters won't be a deciding factor.

Also be aware that there is a re-training effect. If you end up losing a lot of muscle from detraining, your body will remember your previous high point. When you get back to proper lifting, you will essentially get a new period of newb gains until you reach your previous max. So hopefully this will make you a bit less concerned about any potential losses.

As for actual suggestions - I would deemphasize concerns about carb timing or calorie tracking. Just eat to satiation, and do a mirror check every morning to make sure things arent getting excessive. But do ensure you are eating protein at every meal, drinking water and electrolytes, and getting good (enough) sleep. Also, figure out a minimal workout plan that you can do anywhere in order to sufficiently stimulate your muscles at maintenance level. IIRC, this only requires 2 sets taken close to failure 2 times per week. I vote 2 full body days per week with a push, a pull, a legs, and an abs. 1 warm up set each, and the 2 hard sets. Eg, push up progression, pull up progression, pistol progression, L sit or front lever progression - only equipment needed is a pull up bar (find a tree branch or something) and maybe a weight of some sort (a backpack with some waterbottles maybe)