Consider "as well as" in place of "instead of."
It is my exerience that for a very long time we where made to believe that the texts of Ueshiba are talking about spiritual or philosophical issues instead
about using the body in a certain way.
That Ueshibas texts talk aswell
about spirituality as
about certain body work / training methods is something we realize now.
This "aswell as" was not given to us by most of our teachers. Some did. But most did not. The "Tradition of aikidō", the "Dogma" did not.
For Ueshiba himself both aspect where intertwined. For sure! But in a way that you cannot take one without the other, I think. His talking about heaven and earth tells us something about his worldview, but is at the same time something you can practice on the tatami. Two in one. Don't you think so?
(Like in daoistic texts: The body is the tool to "do" spirituality. Aligning the body, breathing, moving in certain ways is good for health, helps to defend against attackers and is a way to become one with nature/universe. This is completely different from view of spirituality that is charcterized by religions based on revelation like Christianity.)
Is a specific understanding of Budo or spirituality necessary to uphold the essential core of Aikido?
I only practice aikidō. But I am a "Neighbour" of some other budō. And a koryū teacher signed a book for me: "True aikidō-spirit is true budō-spirit." I think there is "only one budō" and aikidō does not represent a specific understanding of budō that would differentiate it from other budō.
I don't dare to write about spirituality because I learned that it is understood in a different way in the English language from how we use this term in German (refers to religion, beliefs, being connected to the divine world ...).