You can achieve what you are after (automatic switching between a back boiler and a gas boiler) with the H2 heating panel from here - installed one this year at my parent’s house to replace an ancient Honeywell Sundial system and it works wellMy goodness, this multi-coil, partially vented system was a thought I had as an attempt to keep our back boiler woodburner in the mix while still using a convenient modern boiler, but I cannot for the life of me find a plumber who is capable of even understanding, much less implementing, such a system.
Where did you find your plumber?
They have incredibly extensive install documentation and phone support so even a “normal” plumber could do it