vegas street photography is gonna be amazing! you definitely need something snappy and reliable so you dont miss any shots. i totally get why you hate that 50mm... its just not dependable for fast action. for peace of mind and great results, check these out:

• Samyang AF 35mm f/1.8 FE - this lens is fantastic!! the autofocus is so much faster and more accurate than the sony 50mm. i love the weather sealing too, makes it a very safe choice for travel.
• Sony FE 28mm f/2 - i always recommend native glass for maximum reliability and safety. it is a total hidden gem. tiny, sharp, and the focus is nearly instant on the a7iii.
• Tamron 35mm f/2.8 Di III OSD M1:2 - amazing sharpness for the price! it feels very sturdy and safe in the hand, plus it has great close-up capabilities. im so excited for your trip!! let me know if you need any other help picking gear for your second body.

Coming back to this... are you looking for a specific focal length or is anything wider than 50mm okay? You might want to consider the Sony FE 28mm f2. Its much faster than that 50mm and fits your budget used. Be careful with edge softness at f2, tho. I would suggest testing a copy first to make sure its centered properly... some units have quality control issues.

tbh I have this exact same issue right now and it is driving me crazy. Been looking for a solid street lens for months that doesnt cost a fortune but also actually focuses... its a struggle. A few things I have run into while looking lately:

• everything in this price range seems to have loud motors
• quality control on the cheap third party stuff is hit or miss
• used prices are weirdly high right now I am totally tapped out too so I feel the pain. Before I keep digging though, are you prioritizing something tiny like a pancake for the strip or do you not care if the setup is a bit bulky as long as it actually hits focus?