You seem to be over thinking it. If you want cheap & simple, go with Zodi.
The popular 5liter heaters, Joolca, Camplux, Eccotemp et.al, have higher capacity and more features & safety features.
Primarily on demand ignition, shutoff timer, minimum flow/pressure sensor and adjustment knobs.
As far as cold water performance. My Eccotemp L5 can take ice cold water and get it warm enough for a shower on full flow. If restricting flow with the flame high, it can become dangerously hot if it not understood whats going on and how to operate the appliance.
Its always a good idea to configure a shower enclosure to enable quick escape. To step away, knock the nozzle aside or something.
The temp issue is something I was concerned with. I believe the Camplux has a max temp rise of 75 deg F. I am used to using my Zodi in winter camp and it takes at least 10 minutes or more of recirculating the water back into the source bucket when starting with 45 degree water to get it into the mid 90s so I can get a nice hot shower... I was trying for something I can hook up, turn on, and get hot water in a few seconds like I can with my water heater at home.
You may consider this a luxury, but picture the scenario. You are in camp, it's 40 degrees and breezy, and you've been fishing in the creek all morning. You get up to go make lunch, slip and end up IN the creek.
You need to get out of those wet clothes, warmed up, and into dry gear NOW.
Start a camp fire? By the tiem you can warm up you'll have hypothermia.
Have a change of clothes at the ready in your ensuite tent, and an on demand hot water / shower?
Jump into the Ensuite, strip, shower with a hot shower. Towel off out of the breeze, change into dry clothes, and THEN go and start your camp fire, and wash, rinse those creek water clothes if needed, or just line dry near the fire...
In the same scenario, you can alternatively go with stripping, towelling off, changing into dry clothes, THEN heating water on a stove, mixing to the proper temp, THEN going back into the shower tent, stripping down, taking a hot shower, then towelling off again and dressing in the dry clothes.
The main point is to get warm and dry as fast as possible.