I would suggest being careful before you jump on that Canon RF 50mm f1.8 STM. While the price is great and its awesome for those blurry dog portraits, the 80mm effective focal length is really tight for street photography. You might want to consider how much space youll actually have in Chicago... standing across the street just to fit a subject in can be a real pain. The Canon RF 28mm f2.8 STM is probably the more practical choice for a trip. It behaves like a 45mm lens on your R100, which is much more natural for walking around. Be careful tho, because f2.8 wont give you nearly as much background blur as the f1.8 will. If you can only grab one, the 28mm is more versatile for a beginner, but make sure to keep an eye on your shutter speed in low light since f2.8 isnt quite as bright.

Late to the party but I had to chime in because I've spent way too much money figuring this out over the years. Experience has taught me that the best lens is usually the one that stays on your camera without being a hassle. Like someone mentioned, that 50mm is gonna be cramped for street stuff. Back when I was shooting in downtown Chicago last year, I tried using a long lens and was literally backing into traffic just to fit buildings into my frame. Totally stressful lol. If I were you, I'd look at these:

• Canon RF 28mm f2.8 STM: This is the winner for Chicago. Its roughly a 45mm view on your R100. Super light and fast enough for street, though f2.8 isnt quite as blurry as the 50mm.
• Canon RF 50mm f1.8 STM: Best for those dog portraits because of that wide aperture. The bokeh is way better, but you will find yourself constantly taking steps back to fit anything else in the shot. In my experience, the 28mm is the way to go first. Its tiny so you wont look like a pro photographer, which is actually better for candid shots. Grab the 50mm later for the dog photos once you have spare cash.

Like someone mentioned, that 50mm is definitely gonna feel pretty tight in a city, but I actually want to suggest a different approach because I worry about you running into the same issue I did. I remember my first big trip with a crop camera... I was so excited to use a fast prime, but since my camera didnt have stabilization, I ended up with a ton of blurry shots. The R100 is a great little camera, but it lacks in-body stabilization, and neither the Canon RF 50mm f/1.8 STM nor that 28mm pancake have it built-in. If youre walking around Chicago at night, you might want to consider how hard it is to keep a camera perfectly still. I found out the hard way that even at f/1.8, you still need a decent shutter speed to avoid blur from your hands shaking. I would honestly suggest looking for a used Canon RF 35mm f/1.8 Macro IS STM. It might be a tiny bit over your $300 limit if you buy it brand new, but used ones go for around that price all the time. It has built-in Optical Image Stabilization which is basically a cheat code for beginners. It gives you about a 56mm field of view on that sensor which is perfect for both your dog and street shots. Just be careful with those non-stabilized lenses... they can be really unforgiving if you arent used to them yet. I would hate for you to get home and realize half your vacation photos are soft because of camera shake.