I want to build a wheelset for my Adventure/Gravel Bike based on the DT Swiss GR 531. I want to use my SON Delux Dynamo Hub. Since it is for 24 Spokes I am thinking of mount the rear wheel with 28 straightpull and the front wheel with 24 non-straightpull Spokes. The bike is used during the for asphalt primary but also for bikepacking adventures in Iceland or Norway as well as gravel and forest rides at weekends. My weight is 73 Kg. The bike will probably around 9.5 and I calculate 15 Kg of luggage and food. Are 24/28 Spokes enough for what I want to do?
24/28 spoke count wheels with good quality rims and the correct spoke tension should work fine in this application as your total weight is well within spec for most wheels. With lower spoke count the quality of the spokes and the spoke tension used will be more critical than in a higher spoke count wheel. In terms of anecdotal experience, I have used good quality 24/24 spoke count wheels in bike packing scenario with little issues (I weigh an additional 5 kg to your quoted weights).
Lower spoke count wheels generally will run slightly higher spoke tensions than higher spoke count wheels. Getting the exact right tension is a bit of "Goldilocks" scenario and I will often default to more experienced wheel builders for this key build parameter. The build will be more sensitive to the evenness in spoke tension as there are fewer overall spokes, so take your time during your build to ensure you have evenly tensioned the spokes. If this is your first wheel build you may want to seek out a mentor to check your work. For first builds it is hard to go wrong with 3-cross 32 spoke wheels.
Finally, I hope you consider butted spokes in your build as these tend to result in more robust wheel builds and are worth the extra cost. As the rim deflects (e.g., hitting a hole), spokes can become de-tensioned which can let the nipple back-off changing the spoke tension and putting the wheel out of true. Butted spokes have more give/stretchiness to them than straight gauge spokes, so they tend to handle bigger rim deflections before de-tensioning the nipple.