Unfortunately, the 50mm is going to feel quite cramped for street photography in Japan. It effectively acts as an 80mm lens on your sensor, which is better for tight portraits than busy city streets. I have had issues with using adapters for travel; while the Canon Mount Adapter EF-EOS R works, the added bulk and weight from a lens like the Sigma 17-50mm f/2.8 EX DC OS HSM makes the setup very unbalanced. It was not as good as expected for long days of walking. A more methodical choice would be the Canon RF 28mm f/2.8 STM. It is a compact pancake lens that provides a 45mm equivalent focal length, which is much more versatile for street work. If you require better low light, the Canon RF 35mm f/1.8 Macro IS STM is an alternative, though it might push your budget. Native lenses will simply provide a more reliable experience.

I was in your exact shoes when I first took my mirrorless setup to Kyoto. @Reply #2 - good point! That 50mm is just way too tight for those narrow alleys in Golden Gai. Honestly, I have been super satisfied with the Canon RF 28mm f/2.8 STM pancake lens lately. It basically turns your R10 into a pocket camera, which is a lifesaver when you are walking 20k steps a day in Tokyo. I used to lug around the old DSLR lenses with an adapter, but the weight gets old fast when you're on a trip. This 28mm gives you roughly a 45mm equivalent view on your sensor. It is that sweet spot where what you see with your eyes is basically what the camera captures. For those gloomy Seattle days or Japanese nights, that f2.8 aperture is a massive jump from your kit lens. I have been shooting with it for months now and have zero complaints about the sharpness... it just works. Since you mentioned the gloom, you might also look for a used Canon RF 24mm f/1.8 Macro IS STM. It usually hovers right around $400 used. The huge benefit there is the Image Stabilization (IS). Since the R10 doesn't have built-in stabilization, having it in the lens helps a ton when you are shooting handheld in low light. It's a bit wider too, which is great for street scenes where you want to show the atmosphere. Both of these will keep you under that $450 limit and wont break your back while traveling.

Basically, you gotta look at the 1.6x crop factor as your starting point for any technical decision. An 80mm equivalent like that 50mm you mentioned is technically perfect for headshots but youre gonna be backing into traffic just to get a whole building in the frame in Shinjuku. I remember my first big trip with a similar crop sensor kit years ago. I brought a huge f/2.8 zoom and an adapter because I thought I needed the low light and the range. Honestly? It was a mistake. By day three of walking 20k steps through Tokyo, the weight felt like a brick hanging off my neck. I ended up mostly using a tiny wide-angle prime I had stuffed in my pocket. It made me realize that for travel, portability often beats having every possible focal length, especially on a lighter body. A few technical things to keep in mind for your search:

• Constant f/2.8 is usually the target for gloomy street shots if you dont want to crank the ISO into grain territory.
• Aim for something in the 24mm to 35mm range for a natural perspective that matches what your eyes see.
• Pay attention to the balance; the R10 is light, so a front-heavy lens can make your wrist ache after a few hours. Definitely check out the sample galleries on DPReview or browse some Flickr groups specifically for R10 street photography. Seeing real-world shots at different focal lengths really helps you visualize what you actually need before you drop the cash.

Like someone mentioned, those 20k step days in Japan are no joke. I definitely agree that keeping the weight down is the most important thing when you're exploring a new city on foot. I've been thinking about your situation and honestly, I might have a slightly different take than the others. I'm a pretty cautious person when it comes to new gear right before a big trip. I remember my first major trip I was so stressed about having the perfect low-light setup that I almost bought a bunch of stuff I didn't need. I have been really satisfied just sticking with my basic kit lens and focusing on my settings. It works well and I have no complaints about the reliability. I'd be worried that buying a new lens just weeks before leaving might just add more stress if you're not used to how it handles. Sometimes just mastering what you already have is the safest bet so you can actually enjoy the trip without worrying about your gear... just my two cents tho.

^ This. Also, dont underestimate how much the reliability of staying within the same brand matters when you are traveling. Unfortunately, I had a pretty bad experience trying to mix and match different brands on a trip to Seoul last year. I thought I was being smart and saving money, but the third-party glass just didnt play nice with my camera body and I had issues with the autofocus freezing up at the worst moments. It was honestly not as good as expected and I ended up missing some really cool street shots because the setup was just so clunky. For a big trip like Japan, I really think sticking with the brand ecosystem is the safest bet so you dont have to worry about tech glitches when you should be enjoying the scenery... it just isnt worth the stress.