You need to convert your states, which are strings, to numbers to perform mathematical operations. e.g.
fan_speed: >-
{{ fan_base + mult * (
states('sensor.mvhr_sensors_air_temperature_3')|float(0) -
state_attr('climate.wiser_lounge', 'current_temperature')|float(0) ) }}
The attribute may already be a number (attributes can be any valid type), but better safe than sorry. The state definitely isn’t though. They are always strings.
One final suggestion, stop using device actions. What happens if I have to replace a device - #2 by tom_l