Custom orthoses can help if you have an abnormal foot shape, commonly for example flat footed but if your foot is normally shaped then I don’t see any advantage. If you go to see an orthotist YOU WILL walk out with an orthosis for your sole, whether you have something wrong or not.... it’s their business, and I work with them so I know this all too well.
I have had people come to see me as NHS patients about ski boot problems believe it or not!
I really think that a heel raise is a simple first thing you can try in this instance, you can actually buy them in ski shops in the boot sections to try yourself. However, If it does seem to be of benefit it is imperative that you have a professional fit it properly and measure how much inclination angle each heel merits otherwise you could risk injury.
Also, you will not notice an instant difference and on the first day or two your calves may ache more for the reasons I already mentioned which is that you will not be accustomed to permanently being in “ski forward” position.
I do not believe that stiffer boots would make a difference since even the softest boots should not permit sufficient dorsiflexion at the ankle, most only allow a very small amount with the exception of touring boots which obviously can flex much more.