I would suggest the Canon RF 24-70mm f2.8L IS USM. You might want to consider the weight, but it definitely solves that low light issue you're having with the f4.

Oh man I totally feel your pain on this! I was in the exact same boat when I first started with my gear and honestly it was so frustrating. I took an f4 lens to a friends party and it was just a total nightmare because everything came out grainy and dark... I learned my lesson the hard way lol. It was rough. I honestly think youre gonna love having something faster for Tokyo because those neon lights look amazing with a bit of background blur! Here is what I figured out after carrying way too much stuff for a week:

• Weight is literally everything when you are walking 20k steps a day through Shinjuku.
• Having that extra stop of light makes the photos look so much more professional.
• I stopped caring about zoom range and started focusing on how the lens actually feels in my hand. I actually swapped out my big bulky setup for a much smaller prime lens that I use for literally everything now. It was like half the price of the heavy pro stuff but the images are just way more soulful, if that makes sense? The background just melts away and I can shoot at night without cranking my ISO to like a million. Total game changer. Since you have such a beast of a camera, getting something that lets in more light is definitely the move. Youll be so much happier not having a sore neck by the end of the day!