Create a custom state on the page that has a type equal to a sub topic. When the sub topic is selected then set the state to that subtopic. Then in the repeating group make the data source equal to the custom state’s value.
Are the subtopics option sets or data? Either way I would say you could add qan attribute that is a number called order. Give them a number starting from 1 and going up. Then when you hit next you can change the state to equal whatever option’s order is the current option’s order plus 1. Add a conditional so if that doesn’t exist then it starts over from 1.
Oh and I see those child subtopics appear to be in a repeating group. So when you set the custom state of the subtopic also have a custom state called subtopic index and set that to the current cell’s index. then the next and previous buttons can set the new state of the subtiopic to equal the repeating group list of subtopics item number (custom state + 1 or - 1)
In the data source for the repeating group, you can just put the custom state of the sub topic instead of doing a search But the type of content will need to be the child sub topic and the source will be custom state subtopic’s child sub topics.
On the next button you will put to change the custom state of repeating group subtopic to be repeating group topic’s list of topics:item number custom state index + 1