Normal 4500W water heaters are 18.75 amps. So a "heat pump water heater" that is 19 amps, is actually worse than a normal one LOL.
That's because it is a normal one. Either the heat pump is a lie, or it's an accessory system but it still relies on old-school heating. Well, that certainly complicates the circuit design.
Since it provisions the same as a tanked heater, its heat-pump superpower is irrelevant.
I'm just saying it's a damned shame that you don't have a 120V heat pump water heater that could plug into any random 120V circuit not already dedicated to a room or appliance, and disentangle it from the current (heh) situation. It would sacrifice the 240V heater, or have a simpler, slower 120V backup heater. Which will work. A normal water heater can be wired to a 120V circuit, it'll just take 4 times longer to recover.
The 3-wire problem
Unfortunately, as of the 1980s, it was still legal to run an UNGROUNDED 3-wire connection to a range. In 20/20 hindsight this is completely insane, but it's true. It MIGHT be possible to use this for a feeder to a subpanel, but something needs to be done to deliver ground.
- If the cable used was "/3+ground" black-red-white-bare, you're all set.
- If the cable is the odd "SEU" - 2 black conductors wrapped in a mesh neutral, then it's possible to use this, but you will need to retrofit ground to the subpanel. You will need to insulate the bare neutral so it can't hit grounded bits of the subpanel.
- Your house is too new to use 1960s era "/3 no ground", but if so, see SEU.
- If the cable used was "/2+ground" black-white-bare, this was illegal the day it was installed and cannot be used for the range. There is no way to get neutral out of this and the range needs it.* It can, however, be re-tasked to serve "240V-only loads that do not use neutral", and you get major brownie points if you already know EVs are one of those. For the range, you will need a completely new run of 6/3+gnd from panel to range, or, decide it's a fine time to switch to gas.
So the 3-wire problem will need to be dealt with, if present.
Location, location, location
Your idea of sticking a subpanel inline is a tricky one. You can't just cut the wire in a clever place and squeeze in the subpanel inline, unless you get very lucky with wire routing and have a nice tall panel with lugs from the breaker. Isn't going to happen in a run-of-the-mill 6 space panel, which you can't use anyway because the EVEMS equipment will take more room than that. Note that you CAN mount a panel sideways, but if you do, you can't use breaker positions where down is on.
Further, position of wires tends to be incompatible with subpanel Working Space rules, which require panels to be at a certain height, have a Working Space kept clear of ALL non-electrical equipment which is the width of the equipment (but at least 30"), 78" tall, on the flat not a stairway, and 36" of standback room from the panel face. That rectangular prism needs to be kept clear At All Times. This could be rather ungainly.
However, it may be possible to git-r-dun with load shed devices which are not subpanels and don't require subpanel working space.
"Not all at the same time"
What sweeps to our rescue is the topic of a pair of Technology Connections videos starting here. The idea is that the loads don't need the power at the same time. Now Technology Connections illustrates with the SPAN panel, but we can grab a simpler set of kit.
Required reading is my Q&A on EVEMS and dumb load sheds.
The water heater is a dumb load that needs to be knocked out when the range draws power. You can use relatively simple "load shed" kit like the SimpleSwitch or Blackbox here. The virtue here is simplicity and compatibility with dumb loads; the cost is actually fairly high. But this can happen without a subpanel; you are simply taking the line to the range and splitting it with a Simpleswitch etc. setup to detect current on the range circuit.
I believe the water heater branch coming off the load shed will require a 30A breaker or fuses to protect the water heater circuit. Fused disconnect?
The EV is capable of dynamic EVEMS. That's more versatile and cheaper kit. It has a current sensor on the supply, and figures (capacity - what is being used = available) and tells the car to stay within the "available" value. It does this on the fly in real time.
Since both the range and EVSE are fit for 50A breakers, it might be possible to avoid the subpanel, install a DIN Rail enclosure with the required power meter + a couple of lug splice DIN rail modules, (I'm presuming Wallbox Pulsar Plus here, since an Emporia would be a complete waste in this type of installation)... and not have any breakers involved at all.
After the EVEMS enclosure, you go onward to the dumb load shed that splits off water heater and range.
I think we're getting this done without anything being a "subpanel" that would require working space.
* For the oven light, if you can believe it. So people can use ordinary incandescent light bulbs like everyone uses in every fixture... great. (Clearly, ranges were not designed by HP or Epson. Also 120V is sometimes used for clock and controls. I could not possibly suggest to rewire the oven to use a 240V oven light, and install a UR-Recognized autotransformer to source 120V for clock/controls. Yeah, don't do that.