Managed to lock myself out of this forum by updating my email address but, thanks to Marty Lappin, I'm back in now.
Thank you for some good suggestions - some of which (sidearm, tempering valve, to name just 2) I had to research to find out what they do.
Eldon Yoder's suggestion is the simplest, of course, but I don't really follow why it wouldn't cost much? If I only use the hot water in the workshop - maybe briefly, twice a day - aren't the heating elements going to be working almost as hard to maintain temperature, especially when my HX is ineffective in dormant demand?
What I did discover, researching these suggestions, is that my installation takes no account of the concept of a "thermal block". Across my entire installation, the plate HXs seem plumbed to create just that! In the garage, for example, the plate HX sits on top of the water heater with its HW system exit rising 18" before dropping 22" into the cold intake of the heater. The 18" riser is HOT but the drop pipe is COLD - classic thermal block. Hot water will rise but not sink. The same is true in the main house where the hw exit rises 4 ft. before dropping 10" into the cold intake but there demand is sufficient to keep it flowing and so, while not optimal, it still works.
In both cases, it is only continual demand (or the electric elements) that will keep the water hot since the there is little to no convection caused by the plate HXs. There is a notion I came across of "thermal pressure syphon" that could create a convection process with the plate HXs but the way they are plumbed right now apparently inhibits that.
All in all, from what I have read and You-tubed, it seems a sidearm is the most logical and efficient way to optimize the heat convection without either power or continual demand- and, for low use installs, perhaps the only sensible approach. What do you guys think?
If so, any recommendations on a sidearm? Garage HW heater is older (2,000-ish) and has no side ports so would need to use drain and pressure relief valves about 40" apart.
I do not, it turns out, have a mixing valve anywhere in the entire system. Our water is scalding hot - too hot to hold my hands under - but my wife likes it that way! Maybe she's really from Mars, after all?