How can I mix tank and tankless hot water to feed my bathroom?
Do the simple thing ..
The simplest thing is to add more electric-fuel tank for heating and storing water. You'd do a bit of math to figure out how many gallons you want to be able to consume in a window of time and how much recovery time you'll allow for the heaters to come back up to temperature before the next demand cycle hits. With that one can estimate whether it's even possible to get there from where you're at, and if so, how many tank water heaters you should have in service.
.. or else engineer a solution
You can save a little space and energy by using an electric instant heater to supplement the hot water stored in the tank. You're going off the well-beaten path, though, so a bit of design (aka engineering) will be necessary. Like any custom design, you may find that the design parameters weren't quite right and the solution requires a bit of in-field adjustment.
Following is just one approach of many. This approach focuses on minimizing the load the master bath shower places on the tank heater. It assumes a plumbing topology in which the tank heater continues to directly feed all hot water consumers except the master bath shower. The master bath shower would be fed by the outlet of a tankless electric heater; the tankless heater is fed by a mixing valve; the mixing valve is fed from the tank heater and cold water.

Figure out what is the hottest temperature you'll require at the master bath shower and how many GPM will be needed.
Look at the electric instant water heater's data sheet; find its temperature rise graph. Determine how many degrees temperature rise the heater can provide at the GPM selected in step 1.
Subtract the temperature rise of step 2 from the target water temperature of step 1. This is the required input temperature at the tankless heater.
Adjust the mixing valve so that its output temperature is equal to or greater than the input temperature found in step 3.

This arrangement balances two goals:

provide more water (GPM) at the target temperature than what the tankless heater could do on its own
use the tankless heater to truly supplement or extend the runtime of what the tank heater could sustain on its own