We have a whole house gas fired tankless water heater, at the low flow rates appropriate for washing it doesn't come on.
For the darkroom I have a small point of use tank heater on the hot line that takes care of that problem. It's only 2.5 gallons, so it's easy to run it out of heated water if I'm using a lot of water, but it isn't a problem for normal use. It runs on 110 power, so that makes things easy.
For a point of use tankless heater you would want to look at what its minimum flow rate is for activation. My recommendation would be a point of use tank water heater if power and space aren't issues. Though, the 10 gallon and smaller ones will fit pretty much anywhere.