I've only had experience with the BME280 but should be the same.
Your temperature offset was -0.4 °C which means the sensor read +0.4 Deg when in the ice bath. I would think that is what I would expect from a ice bath. Maybe a little closer to zero but unless you have distilled ice and water and a lot of crushed ice and water in an insulated container, I wouldn't expect too much.
You didn't say what salt you are using. NaCl should be 75% humidity. And it takes a day or so to stabilize.
both were measuring rougly 1% to much WITHOUT offset and rougly 4-5% to much with the right offset
Where are you calculating in the temperature offset? At the raw temp, or after temperature is calculated?
If at the raw temperature, I don't think that is valid as the factory calibration coefficients result in a non linear compensation.