Can a HVAC be split between bonus room and garage, but with return and Tstat in bonus room?
There are multiple ways to do this but since it is a retrofit a 2 head (or more) mini split system makes the most sense.
With a mini split system each room has its own control.
There is no ductwork check them out.
the air in one room can be totally isolated from the other room.
With a conventional air handler the rooms can each have there own intake and output ducts but the air for the 2 rooms is mixed at the handler.
The ductwork gobbles up quite a bit of space and the controls can be in one room but if the rooms are not well balanced the controls will feel inadequate.
Look into a mini split I have multiple zones and they are quite efficient, if you are not using one room turn it off the compressor unit slows down and saves energy.
The pipes and power all fit through a hole in the wall for each room, if the rooms are large or 1 is, that one can have multiple heads if needed and the compressor is sized according.
If you want a central air add on it may cost much more than a mini and take up more space because of the ductwork. The temp control will be better with a mini each room being its own zone. The longer I have my mini split system the more I like it the efficiency is outstanding and comfort is a push of a hand held remote away.