Here is something might make things understanding and how I look at it.
Standard botch mechanics is a die that explodes on a 1 and has chance to botch on a 0 with number of botch depending on circumstance (default 1 but foreign aura/vis, high stress vs some stress, virtues, mastery, etc can modify.). The concept of the stress die is that you can succeed spectacularly and you can also have your hand slip and botch. Its label is deception. It probably should be called Exploding die or Risk Die.
In certain circumstances, you roll stress without any chance of botching. Some of these explicit cases including aging rolls, strength of the effect of certain PeVi spells, mastered spells.
In certain circumstances you roll with greater than normal. Stress can determine the number of botch dice that you have. If you are doing a ritual while your grogs are defending the doors to the building you are in to keep people out while you finish, you will probably have +2-3 botch more than the standard 1 + x for vis + y for aura.
So the fact that something is stress die does not inherently mean under stress.
This is just to establish that just because something uses a stress die does not mean it is automatically stressful. It is important to understand that concept for this argument.
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Rituals spells because of their complex nature and many components have a risk of a botch. Even if you are totally calm and relaxed, your hand can slip. It is not a factor of being under stress, it is a matter that because of the complexity, there is greater risk (understood or not) in casting it. I type at about 50-60 words per minute with few errors when I am in calm situation and get going in my flow but there is a chance my finger slips and I mistype a word. When typing, this is trivial, when casting a spell with vis, I could see how this could be very explosive if it was not correctable or adjusted for.
It is not that I am under stress, it is that precision with complexity is important and slight changes in that precision can get better results than normal or explosing messup.
This is what gives the ritual its stress die and extra botch dice for foreign aura/vis.
Mastery is just that, you have mastered the spell. You have a trained, innate, muscle memory experience with the rituals (or formulaic spell) that if there are no distractions, you will not have any disasters. You might not be so perfect as to get spectacular results but everything will be close to right.
Cooking is example, there are some recipes that I have done over and over for years that the final result is always tasty even if it isn't always perfect (a few minutes off in cooking, spice was off 1/4 pinch) but it will never come out tasting horrible. Until I master a recipe, there are chances of major mistakes, but one I have, I get it right every time unless there is a distraction or stress (phone rings, knock at the door etc).
This is why Rituals (which are like formulaic spells outside the explicit changes like die or amount of fatigue (hey, any spell that is at least 1 hour to cast should impose minimum level of fatigue) use stress die when cast normally and the sentence about no botch dice when mastered and calm applies.
I don't expect this will say anyone fixed in their view on the matter to one side or the other. This is just explanation of how I see it. Once you get off the fixation that 'stress' in the title of the mechanic means it is stress, suddenly a world of interpretation opens up.
As I said, it does not make sense that authors of the game intended for Aegis of the hearth to be major yearly botch chance.