A heat pump is a great and safe way to both heat and cool your shop, but you will need to take steps to keep the coils clean of sawdust. The best pleated filters do well, but need to be blown out or replaced frequently. I use an air gun to clean mine (outside the shop) and replace it about every 6 months. If the sawdust gets into the coils it is very hard to get out and it kills the efficiency of the unit. The mini-split systems do not have the capability of adding good pleated filters, so I suggest getting a regular split system
with the ability to install filters in the incoming air.
My shop is an insulated 14' X 26' outbuilding and I have been heating and cooling it with a window style heat pump. I was fortunate to find that a 12" X 20" standard pleated furnace filter will fit behind the front panel against the face of the coil. The unit has been doing a great job for 12 years now, but it's only run when I'm in the shop or when the outside temps will be below freezing (to keep the plumbing, batteries, and glue from freezing). i have been very satisfied with how well it works.
Charley