Is there a way to slow down these high power chargers? Sometimes I want a fast charge but mostly I want slow to getting hot. I realize overnight charging phones are smart enough to do this but otherwise seems to be impossible without having two different chargers.

The charging smarts are always in the phone, and always have been. Just because a USB port has 15 or 30 or a gazillion Watts available doesn't mean that the phone is required to consume that much. Similarly, the wall outlets in your house may be able to supply a couple of thousand Watts -- but that doesn't mean that a device plugged in is required to use all of that.

And batteries can charge pretty fast these days. Modern pocket supercomputers keep track of battery temperature to keep things within defined limits during charging.

AFAIK the real problem, longevity-wise, for these batteries in normal use is the time spent at extremes of charge (<20% or >80%, ish).

That all said: Sure, some phones have options.

My Samsung phone does some man-behind-the-curtain tricks to attempt to make it reach 100% just before it predicts that I'll unplug it (eg, when I wake up). The idea is to maximize the charge on the phone while also keeping it at 100% SoC for as little time as practical. This probably works great for people with regular schedules (which is to say: people who are not like me).

This phone also lets me explicitly disable various fast-charge modes. I think there's at least two different modes that I can turn off (but I leave them all turned on).

And there's also a mode that limits the maximum charge to 85%, to promote long-term battery health. I have this mode engage automatically when using wireless charging, which is something I only do with the wireless charging cradle on my car's dashboard. (I do want the phone to be powered while I drive, but I don't normally need anything to work extra-hard to cram that last 15% into the battery when I'm on a long drive. It's a good balance, for me.)

It's up to the device. If I have an alarm for the morning my pixel 7 slow charges over the night so that it's full when the alarm goes off. Modern charging standards give all the control to the client device.

You could convert from USB-C to USB-A and back. MicroUSB adapters are popular to chain for devices that only charge on USB-A. There are also adapters just force legacy USB charging for same problem devices. Then your phone will only charge at 12W.

On newer MacBooks you can turn on a similar battery protection mechanism to the phones. I believe vs the phones the smaller chargers won’t stress these cells since they’re a lot larger. 20-40W into a 15Wh battery is more stress than the same through a 52Wh (MBA) battery. A 1C charging rate is healthy.

The device being charged controls the charging current. Typically for smaller gadgets, a single resistor connected to the charge controller IC sets the current. Bigger things like laptops may be more sophisticated.

I use USB-A ports on a dumb charger, but most likely, it'd be best for the battery to use a charger with PPS support (programmable power supply - the device can request finely tuned voltages, reducing losses and thus heat inside the device).

I still keep a few 5 watt chargers around and use them for overnight charging whenever possible. I reserve the use of high power chargers for the odd occasion when I need the speed.

On my Samsung S24 Ultra I have a routine set up where it automatically disables fast charging during the night, but enables it again during the day - the logic being that I don't mind if the phone charges slowly while I sleep, but if I plug it in during the day then I probably want the quickest top-up possible.

If you really want to, you can plug a USB tester (e.g. AVHzY) between your charger and device and manually force the charging mode.

This is why I'm glad my phone has a cooling fan that can be set to come on during fast charging https://i.imgur.com/05lhMJA.jpeg

If you buy a crappy one it'll thermal throttle itself after a few minutes