Beautiful picture!

Reeeeally cool idea! Well done OP!

I think you are searching for the One Shot focus mode. In that way it doesn’t matter where you set your focus point (in this case the center) it should stay there and focus every time you press the shooter button. Like other comment said it, there is more “advanced” ways to personalise the way autofocus works but this should solve your problem for the moment :)

This article explains the focus modes in a easy enough way:

https://www.morethanasnapshot.com/blog/2023/6/7/how-to-use-the-focus-settings-on-the-canon-r50

I hope it helps.