Is the command to bentch peshat or derash?

Summary: Is the command to bentch peshat or derash? Ramban and tradition (and the Karaites!), vs. Shadal. And how to consider it if we accept Shadal's premise.

Post: The pasuk:

10. And you will eat and be sated, and you shall bless the Lord, your God, for the good land He has given you.		י. וְאָכַלְתָּ וְשָׂבָעְתָּ וּבֵרַכְתָּ אֶת יְ־הֹוָ־ה אֱלֹהֶיךָ עַל הָאָרֶץ הַטֹּבָה אֲשֶׁר נָתַן לָךְ:
11. Beware that you do not forget the Lord, your God, by not keeping His commandments, His ordinances, and His statutes, which I command you this day,		יא. הִשָּׁמֶר לְךָ פֶּן תִּשְׁכַּח אֶת יְ־הֹוָ־ה אֱלֹהֶיךָ לְבִלְתִּי שְׁמֹר מִצְוֹתָיו וּמִשְׁפָּטָיו וְחֻקֹּתָיו אֲשֶׁר אָנֹכִי מְצַוְּךָ הַיּוֹם:

As Ramban notes, this is a positive command in the Torah, counted as one of the 613:

(י): ואמר ואכלת ושבעת וברכת -
כי תזכור עבודת מצרים ועינוי המדבר, וכאשר תאכל ותשבע בארץ הטובה תברך עליה את השם. ורבותינו (ברכות מח ב): קבלו שזו מצות עשה, וטעמו ותברך את ה' אלוהיך, וכן ועשית מעקה לגגך (להלן כב ח), וכן ועשית פסח לה' אלוהיך (להלן טז א), ולקחת מראשית כל פרי האדמה (להלן כו ב), ורבים.

וטעם על הארץ הטובה -
כמו ועל הארץ הטובה, יצווה שתברכהו בכל עת שתשבע על השובע ועל הארץ שנתן לך, שינחילנה לך לעולמים ותשבע מטובה, והנה חיוב המצווה הזאת בכל מקום:

They received this as tradition -- קבלו שזו מצות עשה. The reason this is necessary to say, whereas it need not be said e.g. by the command to honor one's father and mother, is that this can plausibly be understood as something that will naturally occur, as a sort of setup for the next statement: When all is well and good, and you eat, and bless Hashem, beware that you do not forget Hashem by not keeping His mitzvot. The vav especially is not helpful, as it seems to indicate another item in the list of causes, rather than the result. But there are a bunch of examples which function in just this way. Even so, perhaps this tradition was necessary in order to keep straight the true peshat in this pasuk.

Shadal, meanwhile, does not consider this to be peshat. Rather:

י ] וברכת : לפי הפשט אין זה ציווי , רק הודעה שכל כך תהיה טובתך והצלחתך , עד כי מעצמך תכיר חסד הבורא ותתעורר לברכו ; ואח"כ הזהיר שישמרו לבלתי שכוח כי מה' היתה זאת להם . כי כן טבע האדם כשתבואהו טובה גדולה , הוא מברך את ה', אך אח"כ ברוב הימים הטובה נעשית לו טבעית , והוא שוכח מצבו הקודם .

That is, this is all a setup of the situation, such that of oneself one praises Hashem. But, one is not being commanded here to praise upon eating and being satiated.

Let us say we accept this premise. What then of the deOrayta? There are two possibilities. One is that this is entirely a mitzvah of Rabbinic creation, much like peru urevu according to Ibn Ezra. That would perhaps make it a derabbanan (but then, with halachic repercussions regarding e.g. safek brachot) or else a deorayta if Chazal can create new deoraytas with their derashot. The other is that this is a mitzvah min haTorah, and always was, but one which was communicated on the midrashic level rather than on the peshat level. But if this is so, I am not certain that we would count it as one of the 613. I believe we only count those that are explicit on the peshat level, but that there are many more deOraytas which are derived via midrash halacha.

On the other hand, who says we need to accept Shadal's premise. This was probably why Ramban took such great pains to establish the tzivuy as peshat. Indeed, let us bring proof from the Karaites. They like dikduk; and they (purportedly) have no interest in establishing derash as the correct interpretation of a pasuk. Yet Aharon the Karaite writes as follows, on this pasuk:

To suggest otherwise was thus not even on Ibn Ezra's radar, and so he doesn't address it. Perhaps someone before Ramban thought about the alternative, but I don't know it with my limited research. Thus, at the very least, it can stand strongly as a plausible peshat.