I can't import my old activities into my Garmin account

I just bought a new Garmin and am really struggling to import my years of activity and health data from my late Fitbit.

I have tried to follow the steps here support.garmin.com/.../ to upload the files from my past accounts but can't make it work for my life.

My old setup was a fitbit I always wore, and I also have all runs and steps recorded on Samsung health. The runs are also saved in Strava, so I could upload from there as well tho I would have to find a way to transfer the steps, heart rate, etc. from fitbit.

I exported my data from google (which now holds the fitbit data) and Strava. The google data is just a myriad of different folders and the few .csv files I could find just didn't work.

I could find more easily the data I want to import from Strava, there is a folder with .tcx files that look to be my activities, but when I try to upload it to connect.garmin.com/.../import-data, it just gives me a message "An error occurred with your upload. Please try again."

Has someone recently succeeded in transferring data from fitbit/strava and could help me figure out what's wrong?