Smart Thermostat without C on Heater
You'll need to make a connection at that quick-connect with the red wire going to it

You see that quick-connect terminal you measured 27VAC from R to? That right there is where the control board connects to C, and can be used as a tap point for a C wire with the correct hardware.  In particular, you'll need a 6.3mm (0.25") stacked or piggyback quick-connect terminal (such as one of these) that accepts 18AWG wire and appropriate crimping and stripping tools.

To assemble this:


Turn the power to the heater off.
Strip the end of the wire (blue is typical, but color doesn't matter) that you want to use for your C wire
Crimp the piggyback terminal onto the end of the C wire
Remove the red wire you measured 27V from R to from its terminal on the control board.  This is the C wire from the transformer.
Install the C wire piggyback terminal's female part onto the control board C terminal
Install the red C wire from the transformer onto the piggyback terminal's male part.
Turn the heater back on, and enjoy your new smart thermostat!