Our Gemara on Amud Aleph discusses the ways it is permissible to write Tefilin on Chol Hamoed. Presumably, this Gemara would seem to prove that Tefilin are worn on Chol Hamoed, as otherwise what could be the pressing need to allow for them to be written? Tosafos (“Rabbi Yosi”) suggests that the Mishna may be written from the point of view (Eiruvin 96b, the opinion of Rabbi Meir and Rabbi Yehuda) that Tefilin are permitted on Shabbos and Yom Tov as well. However, according to those who prohibit wearing Tefilin on shabbos and Yom Tov, it will also be prohibited on Chol Hamoed. Tosafos wrestles with the nature of the prohibition and considers if the issue of having two “signs” applies on Chol Hamoed. What is meant by this is based on the Gemara (Menachos 36b), where Tefilin is called by the verses in Shema as an ois, a sign. And since Shabbos and Yom Tov are also “signs”, that is a cessation from work to honor Hashem, one does not have two signs at the same time. (By the way, this itself is a message about focus and mindfulness. Trying to do too many things at once just cancels out the message.) However, Tosafos raises an objection to this line of thought. According to the point of view that the prohibition for work on Chol Hamoed is rabbinic, what clash of signs would there be that should prohibit Tefilin? Tosafos answers that both the entire Pesach and Sukkos are signified by the Mitzvah of Matzah and The four species.

This Tosafos inadvertently raises the issue of what is the Mitzvah of Matzah on Pesach? The simple reading of the verse (Devarim 16:8) indicates that it is a Mitzvah to eat Matzah all the days of Pesach:

⁦“Six days you shall eat matzot, and on the seventh day shall be a solemn assembly to the Lord your God”

Indeed the pashtanim such as Ibn Ezra note that the verse seems to be saying it is a Mitzvah to eat Matzah throughout Pesach.

Yet, this is a contradiction to the explicit Gemara (Pesachim 120a):

⁦The Gemara comments: It was taught in a baraisa in accordance with the opinion of Rava: “Six days you shall eat matzot, and on the seventh day shall be a solemn assembly to the Lord your God” (Deuteronomy 16:8). Just as eating matza on the seventh day is merely optional, i.e., there is no obligation to eat matza on the Seventh day, as the verse states: “Six days you shall eat matzos,” so too, eating matza during the first six days is optional.

Only on Seder night is there a special obligation to eat Matzah, based on the verse in Numbers (9:11) They shall eat it with matzos and bitter herbs.” 

Despite this the Gra (Maaseh Rav 185) famously held that it was a Mitzvah to eat Matzah on the other days of Pesach, though not as special an obligation as the first night of Pesach.

Many pens have been broken to try to understand this Gra and his sources. In terms of sources, this Tosafos seems to also hold there is some kind of Mitzvah to eat Matzah throughout Pesach, otherwise he would not have been able to explain why we do not wear Tefilin on Chol Hamoed.

Some try to explain the Gra as follows, the Mitzvah of eating Matzah the rest of Pesach is like the Mitzvah of divorce. It isn’t an obligation, but when you perform it properly there still is a fulfillment of a Mitzvah. It is not such a great comparison because arguably the divorce is a Mitzvah to perform properly when separating. So is implied from both the language of the Rambam (Sefer Hamitzvos Aseh 222): “That is that He commanded us - if one wants to divorce - to divorce with a contract under all circumstances. The Chinuch (579) similarly states: “That one who wants to divorce his wife divorce her with a bill (get).” 

However we might actually use this analogy and understand the verse and the commandment in a similar manner. That is, on Pesach, since bread is a basic staple, when you want to eat, you must make sure it is unleavened. Which is then essentially a Mitzvah to eat Matzah as a staple instead of bread. However on the first night of Pesach it is a stand-alone Mitzvah to eat Matzah.

The Arukh Hashulkhan (OH 475 at the end) says it is the will of a Hashem that we eat Matzah all seven days of Pesach. It is an interesting term. How can something be the will of Hashem without it being a Mitzvas Aseh?

There are certain “mitzvos” that are not formal commands but an expectation from God. For instance, we have the directive in Devarim (6:18):

וְעָשִׂ֛יתָ הַיָּשָׁ֥ר וְהַטּ֖וֹב בְּעֵינֵ֣י הֹ

Do what is right and good in the sight of the LORD.

This verse is used as an inspiration for various rabbinically enacted mercantile directives which are about being decent and kind, and not just profiteering (Bava Metzia 16b and 108a). 

This is what Hashem “wants” but it is not a mitzvas Aseh. 

However it is easier to understand this in the light if the extra piety and extra kind acts involved. You could say that since they are judgment calls on how to apply them, the Torah did not legislate them explicitly. But by Matzah, if it was important enough, why not just make it a Mitzvah?

I really don’t have such a good answer though it occurs to me that the Mitzvah of eating matzah is twofold. On the one hand it represents a remembrance that the there was not enough time for the dough to rise and we were redeemed from Egypt in a hurry (Devarim 16:3), but also a remembrance of the slave rations in Egypt (ibid, “lechem oni”). Perhaps the obligation to eat Matzah specifically on Seder night is about the redemption. However on the other days of Pesach it is more an obligation to NOT eat chametz, which is fulfilled in a positive sense by eating matzah. This second aspect of the Mitzvah is just remember the slave rations. Because it’s remembering an unfortunate aspect, God did not want to make it into a constant seven day long mitzvah, in terms of a specific obligation.