What am I doing wrong with the Deathcare?
Dead bodies are contantly piling up in a city with around 55k population, I have something ridiculous like 50 crematoriums and 30 cemetaries, but no hearses are ever deployed, I have tried decreasing education to create more uneducated workers but that doesn't fix anything, I have decongested the roads, but that also doesn't work, for some reason the hearses are never deployed so dead bodies constantly pile up, I even had to manually destroy and rebuild a few universities because of this.
It is a demographic problem. If your city growth with a constant rate, the people will die with a constant rate. But when you expand your city in waves the people will die x years after they moved in, in a similar same wave.
Try using the de-zone tool to demolish a lot of residential at once while leaving all your roads and services and stuff intact, then slowly rezoning it.
Let each bit develop for like a week or two, and then zone a little more and repeat.
I fell into the same trap once I had a lot of money and starting zoning larger. That's how I fixed it for my city.
More than that though, out of all the problems that real mayors face it is highly unlikely that dead body removal will become a serious concern that spirals out of control (unless of course a war or epidemic broke out).
OP, trying moving the death facilities or turning them off then back on.
I was having the same issue with garbage trucks and the incinerator and turning it off and on at the actual facility fixed it.
but you also probably have traffic issues, virtually everything in this game comes back to traffic issues, right down to how much money you make at any given moment.
if your people can't get to work, they don't make money, if they don't make money, their work doesn't make money, if tourists can't get in your tourism doesn't make money, etc. this game is almost all about traffic management. it's funny because the simpler your roads, the better the traffic. a single city of just two side by side long roads with a connection at the end, has basically perfect traffic flow.
a quick fix would be to bulldoze homes here and there in some semi random order, so that their life cycles reset and space out to be more managable.