Gonna leave my two cents. While this story was promising in the beginning, despite it's quality technical writing and all around competence of the writer, I can't call this a really good story. It has good and consistent characterization for its main characters (with one glaring exception), it keeps up well with FGO more lighthearted moment, Mash is genuinely endearing and Ritsuka works as protagonist offering a nice contrast to the servants and Romani, Da Vinci. Mash and Ritsuka have genuine chemistry, Cu is interesting and Herk was just great. Nobu and Okita are on point, and you grasp their, and FGO events, brand of humour very nicely.
The main problem I have with this, however, can be summarise as: unfocused. It tries to do multiple plotlines at the same time, and even though they are not serious plotlines, they demand a lot more focus and thinking than what's been given to them, it plays around with humor, day to day shennanigans, some sweet romance, without actually commiting to/exploring any of those things in any significant manner.
Adding Kanna was an unwise choice, as she is now. She adds almost nothing to the story, her only role so far has been to serve as a contrast to Ritsuka or rehash some of Shirou's old plotlines. She's supposed to be charismatic, but we don't see that. Ever. The servants already like her and she does nothing you justify that. It's just stated as a fact and taken for granted. She's quirky? Okay. And what's the result of that? Nothing. In summary, Kanna is boring. Nothing she does or happens to her is interesting, new or relatable. Her issues aren't issues we can relate and she has very few moments where she changes things or is changed. I know what you wanted to do with this, the twins were supposed to be light hearted antagonistc contrasts that complemented each other. But its badly applied. The story would be better without her, by quite a bit, since she only detracts from the focus on better characters.
You reuse your favorite characters like Cu and Nobu/Okita and Archer over and over again in identical or nearly identical situations. I think this is pretty direct. These guys keep doing the same things over and over. It gets boring fast. Cu is the one that suffers the most, since he is never the main agent, he's always there to react in a Cu manner, which we can think 'oh, that's so Cu' and then promptly ignore him.
New routes or new Characters are rarely explored and a lot of time is spent shilling on the main characters to compensate the fact that we are never seeing them doing anything. Shilling, I have to say, while a minor problem is that one that annoys me the most. You spend so much time, telling us how X or Y is great, because of something or other, so much time, and you even repeat yourself, for no reason. We're never shown this competence, so it begs the question, if it doesn't matter enough to be shown, why talk about it? The most notorious example is how D'eon first PoV chapter is one long inner monologue shilling the main characters. It's D'eon chance to shine, and yet we finish that chapter knowing new nothing about them. The most constant shilling though, that baffles me, is Shirou/Rin/Artoria shilling. Like, you assume that we know FGO enough to start from the point that Mash and Ritsuka love each other, but not enough to know well the FSN protags?
These aren't structural problems. Just punctual problems that to me show that the author kind went on automatic when writing this. This isn't a bad fic, far from it, just unambitious and frankly, a little lazy at times. I feel that if you, the author, sat down and decided to do something really good, and drew an outline and commited to it, you would definitely do it. You got the knowledge and ability for it.
