Saw this a day late but I had to jump in. I went through this exact struggle when I first started out with a crop sensor body. I ended up grabbing the Canon RF 50mm f/1.8 STM for a family gathering and quickly realized how much it changes your workflow. I was literally backing into trees and bushes just to get a decent waist-up shot because of that 1.6x crop factor. It turns that 50mm into an 80mm lens, which is brilliant for portraits but can be extremely tight for social events in small spaces. I would suggest being very careful with your framing in a crowded park. You might find yourself constantly asking people to move so you can step back far enough. Also, watch your budget because that sub-$200 price point doesnt usually include a lens hood, and you really need one to prevent glare if its a sunny day in Philly. My quick tip is to practice your distance before the big day... go to a similar space and see exactly how far back you need to stand to fit a person in the frame. Another thing, make sure to set your shutter speed a bit higher than usual. Since that lens doesnt have image stabilization, any slight hand shake at that tight focal length is gonna result in blurry shots that you wont notice until you get home and see them on a big screen.

I would suggest being quite careful with your choice here because the crop sensor on the R50 changes the behavior of these lenses quite a bit. Since you are on a tight timeline for the graduation in Philly, sticking to native glass is definitely the safest path to avoid any weird compatibility issues with adapters. You might want to consider how much physical space you actually have in that park before committing to a focal length. I have used both of these and here is how they stack up for your situation:

• Canon RF 50mm f/1.8 STM: This is basically the gold standard for budget portraits. It will give you that blurry background you are after, but remember it acts like an 80mm lens on your R50. You will need to stand back about 12 to 15 feet for a decent portrait. Make sure to watch your focus carefully at f/1.8 because the depth of field is very thin and you might miss the eyes if she moves.
• Canon RF 28mm f/2.8 STM: This is much wider and easier to use if the park is crowded. Tbh, it wont give you that heavy bokeh look as easily as the 50mm does. Its a very reliable lens, but for specific portrait work, it might feel a little too similar to your kit lens at the wide end. Personally, I still think the Canon RF 50mm f/1.8 STM is your best bet for those pro-looking shots. Just be careful not to back into a fountain or something while you are trying to frame the shot. Its really the only way to get that real depth on such a tight budget.

i want to address the space issue. i remember taking a similar setup to a graduation party and i was physically constrained by the garden walls trying to get a decent shot. it demonstrated that while the longer focal length is great for blur, you really need about 15 feet of clearance to make it work reliably... it wont be easy in a tight park.