I agree with others... tripod should be the priority here. If the owners pay you to get good photos, they should give you time to set up and get the best shots possible. If they just want quick jobs they can just take photos with their own phones. You should also be arranging furniture, decorations, playing with the lights etc. - this is where your art will shine. And all of this takes more time than setting up a tripod.

A new camera definitely has better ISO performance, but since this is indoors and no action, just do a long exposure shot at ISO 100. You'll get a clean photo. Also, do some exposure bracketing and HDR manually in post. Obviously, your composition is key here, so a wide angle lens would really help a lot.

I'm sure this way you can get much better results compared to 'point, align, and shoot automatically' even with a new RM10K camera and RM5K lens.

If you can't get good sharp photos with Sony A6000 + Rokinon 12mm f/2.0 then it's no longer gear problem then

A slightly cheaper option would be Sony A5100 + same lens. Exact same picture quality. The advantage is the screen that can be tilted up. Useful for shooting from corners. Actually, for the manual focus Rokinon 12mm, you can get just to infinity focus and most likely everything will be sharp. Wide angle lenses have deep depth of field so autofocus is not really an issue for you.

About the ability to zoom...you mentioned you only post online on property sites. Those photos are not very high res. A6000/A5100 has 24 mp, plenty for you to crop.