Increasing your frames depth to 24' is certainly doable, but will almost certainly require resizing/spacing your attic floor joists (possibly switching to an engineered joist).

It's impossible to advise on rafters without knowing your local snow load requirements. If that load is high it may make sense to add a pair of posts to support every beam and extend that up into to the attic to support mid-rafter carrying beams.

Ultimately your best bet is to get a timber frame engineer to check all your calculations and discuss options with you. And you'll probably need to have one sign off on your final design to satisfy your local zoning and planning officials.

IIRC aside from splinters, the biggest objection to hemlock is the possibility of ring shake. Most people will avoid the gamble that a cut timber starts coming apart as it dries as they aren't following it from growing tree to finished beam.