Welcome to the forums
I'm sorry to hear you've been laid off from work. That's unfortunate... I hope something suitable comes along in the near future.
On that basis, though, I wonder if now is the right time to spend money on lenses? Obviously it's your decision entirely, but the DA50 f/1.8 is already an excellent portrait lens when used effectively. On your K-S2, it gives a very similar field of view to what you'd get using the FA77 on the full-frame Pentax K-1 or K-1II. Plus, the image quality is (like most modern 50mm lenses) rather good. It needs stopping down a little to become sharp in the mid-frame and edges, but is actually quite usable even wide open for head-and-shoulders portraits. Plus, at f/1.8 max aperture, it's pretty fast, which is great for controlling background blur.
The DA*55 f/1.4 will undoubtedly give you better results, and a field of view very similar to the classic 85mm on full-frame. It's faster, too, and whilst you'll still want to stop down a little for best results, you'll get those results at wider apertures than your DA50, allowing for even greater background blurring.
The FA77 is widely considered to be the best Pentax prime lens currently available, but you need a bit of room between you and the subject to work with it. The field of view is considerably narrower than your DA50 and the DA*55. Outdoors this won't usually be a problem, but be aware that indoors it might be limiting.
Being a zoom lens, the DA18-135 is the most versatile - but at the focal lengths you're most likely to use for portraiture (50mm and upwards) the maximum aperture might be something like f/4.5 or worse, which isn't great for controlling background blur - something I'd suggest is quite important for this type of shooting, depending on the background. Plus, you'll need more light generally. Of course, neither of these is a problem if you intend to shoot exclusively in a studio with hanging backgrounds and lighting.
In your situation, I'd personally recommend using your existing DA50 to the best of its (and your) ability, as you really can do so much with that lens for portraiture. But if you're certain about wanting to buy another lens at this point, ask yourself whether you're ever likely to move up to full frame in the near future. If not, then I'd suggest the DA* 55 might be the most versatile choice for portraiture. The FA77 will offer the most pleasing images, but only if you have room to work with it