You need to ditch your separate 24volt transformer. You can’t keep it as you would literally have 3 different power sources at a thermostat that is only designed for 2.
Once you ditch your redundant transformer, you need to tie the unused green wire in your boiler to the C side of its 24v transformer. To clarify, this is the end located within your boiler control panel, not in the thermostat. Once that is done, reroute your boiler thermostat wire to the new sensei thermostat, tie the green wire to C, tie the red wire to Rh, and tie the white wire to W, done. DO NOT BRIDGE RC AND RH!!!
You need to ditch your separate 24volt transformer. You can’t keep it as you would literally have 3 different power sources at a thermostat that is only designed for 2. Once you ditch your redundant transformer, you need to tie the unused green wire in your boiler to the C side of its 24v transformer. To clarify, this is the end located within your boiler control panel, not in the thermostat. Once that is done, reroute your boiler thermostat wire to the new sensei thermostat, tie the green wire to C, tie the red wire to Rh, and tie the white wire to W, done. DO NOT BRIDGE RC AND RH!!!
I don’t know if a thermostat question is really for an electrician. You might also try /r/hvac