My steps are counted twice or are much higher than my watch shows

Two apps or devices probably send steps for the same day, for example your phone and your watch. Keep one main step source, then send support a screenshot to fix the days already counted.

Android: open Health Connect → Data and access → Activity → Steps → Data sources and priority. Put your main app at the top and remove other apps that send steps.

iPhone: in the Health app, go to Browse → Activity → Steps → Data Sources & Access, tap Edit and move your main device to the top.

Then send support a screenshot from your main step app with the date and the daily total for each wrong day. Fix the source first, or the next sync can bring the wrong number back.

With a Garmin watch, you can choose Garmin as your integration so steps come only from the watch.