I was going to suggest one of those meters lol! I figured I was already rambling and he would just be starting out anyway. I do use one and I really enjoy being able to check out my levels, like you said it's not a necessity but it's definitely interesting.
Yeah, it also depends on if your glycogen stores are actually full, which, if you are obese, they probably are. Retatrutide should actually help with that as the glucagon component stimulates glycogen release from the liver which is cool!
And yeah protein can absolutely make a difference, my husband does strict carnivore, and while I have a little more difficulty sticking with it, we noticed I generally have higher ketone levels than him, because he eats more protein.
100 grams of carbs regularly is crazy! Your system just starts clearing it out so fast. When I started out doing keto I'd give myself one cheat day per week. I kept doing it for a while, and I noticed that once I had been fully keto adapted, the cheat day would make my ketone levels drop from approx. 6 mg/dl to at worst 2 mg/dl, which is still definitely ketosis lol. It would go right back up afterwards too.
You're definitely right about that last part too, insulin inhibits lipolysis and ketone production, so if your baseline levels are increased, it's gonna be harder. Insulin resistant people also have a metabolic preference for glucose over ketones, so it's going to be an absolute last resort, and probably inefficient, so definitely bad and long lasting keto flu. Thankfully GLP1s help with this transition though!