If you have an extra pair of cycling shoes, I would try a large cutout over the bunion. A big hole.
Are the soles of your cycling shoes wide enough for your feet?
I used to get various foot pains until I switched to Sidi Mega width shoes and now I have zero foot issues. Probably not going to help you but mention anyway. I had hot foot and the area that you show would get really sore and hurt a lot but I did not have bunions to my knowledge.
I still contemplate custom cycling shoes but the cost is high. You might ultimately need to go that route because I am unaware of any sandals that take pressure off yout bunion afflicated area. I just do not think this is the solution for you
If this 600K is a PBP qualifier and if I were you, I would get my hands a couple pair of preferably wide shoes and start cuttin.