[polyglot]

The density after the first dev was in my opinion "acceptable" as a B&W negative, but does it have to be much denser in order to be able to remove the silver slat that the second developer will "see"?

Yes. A negative is of low contrast (typically 0.7ish) while a positive wants a contrast of at least 1.0. Remember also that a neg has bucketloads of highlight latitude, which means that there is a significant quantity of silver remaining in the emulsion that was not developed.

To get a slide with clear highlights, you need to reach the absolute shoulder of the film curve where additional exposure or development will produce no additional density - at that point, you've developed all the silver into metal in the first developer so that there is nothing left for the second-developer to act on. And if you want black shadows, there will need to be clear parts on the negative.

My guess is you need lots more first-development, perhaps more exposure.