Honestly, if you have 300 bucks and a gig coming up, forget the adapter headache. Just grab the Canon RF 50mm f/1.8 STM. It is basically a no-brainer. The optical design is tweaked for the R mount, so you get way less chromatic aberration wide open compared to the old EF version. Plus, the STM motor is super quiet and precise for eye-autofocus, which saves my skin when I am shooting portraits. If you want to spend the rest of that budget on something actually useful, look into a decent variable ND filter or maybe a cheap godox speedlite if you dont have one. Ngl, having a dedicated RF lens is just a much better user experience because it balances perfectly on the smaller EOS R body. You wont be dealing with extra length or weight, which makes a huge difference during a long session. I know it seems like a simple lens, but the sharpness is surprisingly good for the price point. If you want something with a bit more reach for headshots, you could try to hunt down a used Canon RF 85mm f/2 Macro IS STM, though it might push your budget a bit. Either way, dont overthink it. The native glass is just built for the sensor, and for 200 bucks, the nifty fifty is tough to beat for paid work.

Been thinking about your situation since I saw this earlier. Using an old EF lens on the R system can be pretty hit or miss. I remember dragging my Canon EF 85mm f/1.8 USM through a wedding last year using the adapter, and while the optics were sharp, the weight distribution felt super front-heavy. It made my wrist ache halfway through the ceremony, which is something you definitely dont want during a paid gig. Here is how those options stack up in my experience:

• The Canon RF 50mm f/1.8 STM is tiny and honestly creates a really balanced setup on the R body. The autofocus is near silent, which is great if you are shooting video too. Just be careful with the focus speed in low light; it hunts a bit more than the L series glass.
• Finding a used Canon Mount Adapter EF-EOS R is fine if you have a bag full of EF lenses, but for just one lens, it adds an extra point of failure. If the pins get dirty, you lose communication with the camera mid-shoot. If I were you, I would just grab the native RF glass. Dealing with the adapter shim and the extra bulk just creates more stress before you even start the shoot. Make sure to update your firmware before the gig though, sometimes the older bodies get a bit picky with the new RF primes.

Honestly, just pick up the native glass. I had a really frustrating time trying to get a used adapter to work reliably during a shoot last month, and the autofocus was just not as good as expected when things got hectic. Unfortunately, dealing with extra pieces between the body and lens is just one more thing that can fail on you.

• The native RF mount is way more stable
• You wont have to worry about connection drops
• It fits your budget perfectly If you really need something for portraits, the RF 50mm is basically your only safe bet under 300 right now. I know it feels a bit basic, but itll get the job done without giving you any weird adapter errors mid-session. Good luck with the gig.