70 degrees is about twenty degrees too cold.
Which suggests either a lazy start, or you simply aren't allowing the car to heat up.
Setting the heater to the coldest setting will help, as will unplugging the fans... Just keep a close eye on the gauge whilst the fans are disconnected.
Fundamentally though, it sounds like you need a new thermostat. It won't go closed loop until it's up to temperature.
won't turning the heater to coldest bring on the a/c fan ? cooling the rad
surely better to turn off the Climate controls altogether ,so the HBV closes
seems very odd that the engine won't get up to 92 plus degrees when standing ,bonnet closed ,idling
wondering if there is a blockage of gunk from the blown HG,or air lock on the coolant bridge
does coolant pour out if you unscrew the coolant temp sensor Mr Addy ?
that said ,the dash sensor is showing too cold ,but that's on the coolant bridge too
sorry to hear you've not been well ,hope YOU and the omega
have a speedy recovery get well soon
one thing you could try ,plug the old coolant sensor in the loom ,engine running,look at live data and warm the sensor up manually (lighter ) see if you get 90+ and closed loop etc
obviously taking care of moving parts etc