How to fill in exterior paint on plywood
Additional coats will not help much. The key would have been to use a heat gun or chemical stripper to remove all the old paint, or sand the areas where bare wood and remaining paint meet until the surface irregularities were minimal.

You could try using a taping knife or wide putty knife and spread a layer of exterior spackle across the whole surface, then sand smooth. This is a bit of an art and if you aren't practiced it could end up being a lot of trouble/work. If the paint you already applied has a sheen it would not stick well anyway.

I think you know the answer to your question... it was your last mentioned option (except I think you should consider a heat gun).

Note: higher sheen/gloss paints will tend to exaggerate surface irregularities whereas low sheen can help hide.