My house has the toilet and shower branching from the same cold water line. This means that whenever the toilet flushes, it uses a lot of cold water and the shower gets too hot.
To solve this I found the valve for the toilet and closed it just enough to fill the toilet without making a weird noise. This reduces the amount of water the toilet uses when filling, which means the shower doesn't get super hot.
Because of how toilets work, flush performance is unaffected, and my showers are now a lot less perilous. I'm a little ashamed it took me so long to figure this out, but it works so I thought I'd share it.