I don't think the epoxy by itself would be a long term fix for the longer cracks.
I would go with a fiberglass repair kit that you can get from any auto repair store. You can use the cloth+resin to strengthen the cracks.
For the hole in the corner (your first picture), maybe just some automotive body filler. They come in several grades, depending on the amount and size of the reinforcing fibers in the mix.
Of what I know about fiberglass, you will need to grind through the gel coat, which is the smooth color coating over the outside of the board, down to the layer of woven fabric that is under the gel coat. feather out the area a few inches on either side of the damage, being careful not to go into the core. If you do a little, no big deal, it just means a little more work to fix. When grinding fiberglass, it stinks and dusty, you will want a good respirator or at least a dust mask with a rating 95 AND do this outdoors.
Cut the fiberglass mat to fit over the dressed areas, and set aside. With the larger damaged section, you will need to cut it off and rebuild the core material before cutting the fiberglass mat. DO NOT use bondo for that or any repair on this board! It is not structural at all for this application, and it may damage the existing core material. Use the same material that has been exposed to rebuild the corner. You will need to research what adhesive it will take to apply the repair piece.
With all the fabric cut to size, mix the epoxy. It has a very limited time to work with, so you will need to move efficiently. Coat an area that has been prepped, you may want to mask around the area so the epoxy goes where you want it, not everywhere else.... Immediately, place the mat set aside for that area over the repair and add more resin over it. I forgot the mention, When the cut is made on the worst part of the damage, asses how thick the thickness of the fiberglass is. You will need to add enough layers of new fiberglass mats to build it up the same as original, alternating resin and matting. Make certain that there are no trapped air bubbles in the layers. Working with narrow strips of mat in all areas, will help the material conform to the roundness the board has.
After all the matting is done, let it cure for the recommended time and sand smooth. You can apply a layer of epoxy colored the same as the board. Whether it will need one coat or 2 of the color, I do not know, but it will help make the repair not so noticeable... perhaps....