Assuming you will be using silicon-steel laminates for your transformer design, below is a link dealing with this material.
"Among all the soft magnetic materials, silicon steel is still by far the most commonly used material with an annual worldwide production of ten million tons and a market share of 80% [30]. Silicon steel (3.2 wt% Si) is currently the most popular choice for motors and transformers because it offers balanced electrical and magnetic properties"https://www.sciencedirect.com/science/article/pii/S0304885318331330Some other things that you will need to consider in your design are:
1/ Allowable flux density (Tesla) in the core against core losses.
2/ Core volume needed for specified output power.
3/ Winding volume needed on the bobbin. (fill factor)
4/ Allowable temperature rise against maximum ambient operating temperature.
5/ Balance of core and copper losses.
6/ Current density allowed in winding wires. (will affect losses and temperature rise)
https://eepower.com/technical-articles/how-to-design-a-transformer-size-and-selection/#