As you are learning, stainless is a tricky material to work with. Also the specific grade of stainless really makes a difference in your experience (e.g. 316 is MUCH worse to machine than 440).
Sharp tooling is your friend when it comes to stainless. Some general purpose inserts are too blunted (intentionally) to form a nice chip without tearing in SS. Try some HSS, or select material specific inserts for SS. I have even used aluminum specific inserts to machine stainless due to their very sharp edge (the tool life is not great, but was fine for my needs).
A 12x36 should give you enough rigidity to take a healthy cut, but you probably won't be ripping through material like Abom79. I'd start with perhaps a general purpose CCMT insert for roughing, then a CCGT or HSS for finishing. Try HSS for threading too. There are extra sharp carbide threading inserts out there if you know what to look for. Play with the spindle speed and feedrate setting on the gearbox to dial in a cut you can keep the machine running all day. Then adjust your depth of cut as necessary to keep the spindle nicely loaded during roughing. Coolant is recommended. I personally like a squirt bottle full of synthetic coolant (mostly water) used in CNC machining centers rather than oil (no smoke).
