Since the first page seen property is an analytical property updated automatically there's not a way to remove the utm parameters from the report results and aggregate by only the root of the url. If your team has Data Sets which an Operations Hub Enterprise feature, you might be able to create a formula that reports the results as you need.
The other recommendation would to be to sort by the first page seen property so atleast the urls are in order and you could group them by page visually. The utm parameters would still be present. You could also export the results and further organize the results if needed.
If you'd like further flexibility in using analytical properties in reports, we invite you to the topic and your use case on HubSpot's Ideas Forum so the Products Team evaluates for future roadmapping.
Aggregate First Seen Pages (removing utm parameters) in Reports
@FGhermandi, happy to clarfiy here! With Operations Hub Enterprise you have access data sets which allows you to manipulate your data further and create further data sets. It would require understanding and testing out of the forumula syntax available to see if this resolves the results you expect.
I've not personally seen this use case. You could consider reviewing this KB article in full and then trying a free trial of Ops Hub Enterprise to test out the feature.