Call an HVAC company.

Motels have some great wall hvac units that are so quiet and work really well.....possible solution with heat lamps in bathroom. I imagine that the units are designed for various sized areas?

My first suggestion is to inspect the duct work between the master suite and the furnace. If the duct is running in an unconditioned space, is too small, or is leaks, then there is a chance it could be fixed and solve the problem. You may be also able to balance the air flow so more of it will go to the master suite.

If duct repairs and balancing don't resolve the issue, then look into adding a mini split. You may be able to heat the whole master suite with one cassette. You may need a second cassette for the bathroom.

I suggest getting advice from a HVAC expert! When we had our new HVAC installed, we spoke in length with them as to how to get adequate heat and A/C for our attic room since it is very cold in the winter and sweltering in the summer. They told us all that would be involved to get heat and A/C to that room, which included duct work.

There is way too much info missing to make any evaluation.

Location for design temps? Vented/unvented crawlspace? Air sealing? Insulation in crawl, walls, and attic? Flat or sloped roof? How much glazing? Addition? Existing HVAC sized correctly (BTUs and duct sizes) for house? HVAC resized for addition? Location of ductwork? Returns present? CFM, airflow and temp difference?

Find a competent HVAC contractor. Note the difference between companies - some just service equipment; some just replace equipment the same and use rules of thumb (like 400sf per ton); some will actually do Manual J calculations and investigate all the items I listed above. You need a company that can do the latter.

I should edit this to say probably two companies: an HVAC one and an energy consultant. Many power companies provide the energy consulant for free.