Mortal sin is, by definition, the loss of all grace.
You have said that to think of missing Mass is the same mortal sin as to actually miss Mass - the loss of all grace, and the certainty of Hell, if a good Confession isn’t made, in specific kind and number, prior to death.
I did not say “to think of missing Mass is the same mortal sin as to actually miss Mass” Never. I carefully said it is a mortal sin for example to decide to not go to Mass (without excusing reason)(though one can commit a mortal sin in other ways) if such is done with full knowledge and complete consent. Yes of course. This is how this is understood in the Catholic Church. But It is not the “same mortal sin” as missing Mass…actually missing Mass (without excusing reason) would make the sin worse. And yes the person is in danger of hell …for they may not make it to confession or have a perfect act of Contrition etc…before death.
You have said that the thought alone is, itself, the full and deliberate consent - no action is required. If you have a craving for chocolate, you are already a glutton, and already in the state of mortal sin, even if you never go out and buy any chocolate.
Yes one can commit a mortal sin “in ones thoughts” in various ways. Mortal sin in thought or action or omission or desire…still needs the three things…grave matter, full knowledge and complete consent. and one needs to understand what is meant by thoughts here…and also what is temptation and what is sin.
The example you give here is “objectively” more in the order of “maybe a venial sin” if eating the Chocolate would be contrary to reason. Over eating…is ordinarily objectively VENIAL MATTER not grave matter…so one would not want to use such as an example of mortal sin (objectively)…If I was to crave chocolate and was on a diet…and was to consent to such a desire…there can be no sin at all for me…or perhaps a wee venial sin…but certainly not mortal…for I am very aware that such is not grave matter…It is not going to cause some grave injury to my health in the short term.
Gluttony and gossip are both extremely grave sins.
Gluttony and gossip (or detraction) can be grave matters. Gluttony for example in if one were to choose to get drunk and thus loose ones reason…and detraction if what one reveals without objective reason is a grave thing…or would do grave damage to them etc…
Not even if, in your daydream, you play the role of a knight?
If I day dream that I am a knight…I would of course only daydream of acting as t just knight

…but no…just like watching a movie I enjoy the trill of the bank robbery…such is not willing to rob banks…such is not sinful…it does not led me to sin. This is fiction.
Though of course sexual matters in this sort of thing is a different case…