In general, shutter speed is your first choice to prevent blur, both for the camera and the subject. IBIS can help with camera motion, particularly when you cannot use a faster shutter speed. It also helps us older photographers with hands that aren't as steady as they used to be.
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@drmikeinpdx has some experience in the boudoir realm. He may be able to offer some recommendations here. Check out his prior forum posts and work at
https://www.beyondboudoirphoto.com/.
Depending on your situation, you may find a single 50mm lens somewhat limiting. I've taken portraits in a variety of situations for family and organizations that I'm involved with. I started with an FD 35-105 back in my film days, then transitioned to the EF 24-105 f/4 L before moving to the faster EF/RF lenses. Depending on the location, lighting, and background/bokeh I want to achieve, I use a fast 85, 135, 24-70, or 70-200 these days - shorter lenses indoors, longer lenses outdoors. I never had the need for a fast 50 in my shooting - I almost always had enough room to step back and use the 85. It really depends on the perspective you are trying to achieve.
My portraiture has improved more through technique and practice than specific lenses. I shot both handheld and with a tripod. I often use a remote shutter release with the tripod so the subject looks at me. I found that it can give a more natural expression than looking at the camera.