Attackers have made extremely good models of mangling that humans perform on passwords, so this kind of change just adds whatever bits of entropy there are in y

Attackers have made extremely good models of mangling that humans perform on passwords, so this kind of change just adds whatever bits of entropy there are in your selection of mangling approaches. It's almost certainly less than you think. It might help to understand what people have done here: There have been absolutely gigantic plaintext password leaks. Armed with plaintext passwords and collections of very good 'dictionaries' (might not actually be dictionaries) attackers design rules to turn the dictionary into as many passwords as possible without duplication.