Wonder if Sabesto (nick on github) the developer of Flexit A/C Controller and pyflexit python library is on this forum?
Looking at the climate flexit controller and lib - it can be quite easily transformed into the generic modbus climate component. As many ventilation units use modbus - this will be great. Not a tech guy - but looking at the code, all is needed is to move the hard coded register addresses into the HA configuration file so a user can set the register manually.
Of course some modification might be needed also on the payload expected - but…
As an example in pyflexit/pyflexit/pyflexit.py
'Heating': {'addr': 15, 'value': 0},
as i understand the Heating expects either 0 or 1 but in systemair it can also be 2
0: no heater
1: Water heater
2: Electrical heater
3: Contactor
But even starting with basic functionality of fan/speed controll will be great. Most registers are easy to transfer
P.S. This is a systemair manual for the modbus Systemair modbus PDF