To add to the point above: I have to respectfully disagree with the idea that you need to spend the whole 800 bucks on one heavy zoom. In my experience, mobility is king when you are navigating the Tokyo subway. I have tried many setups over the years and the Sigma 18-50mm f/2.8 DC DN Contemporary is just unbeatable for the price and size. If you really want that extra reach tho, dont sleep on the Sony E 18-135mm f/3.5-5.6 OSS. The image quality is actually surprisingly sharp in the center and it handles the a6700 sensor just fine. Tbh, I would grab the Sony used for like 450 and spend the rest of your budget on a tiny low-light prime like the Sony E 20mm f/2.8 for Shinjuku nights. It is way more practical than lugging that massive Tamron around all day, trust me.

Honestly, I went through this exact same dilemma before my last trip to Japan. I ended up taking the Tamron 17-70mm f/2.8 Di III-A VC RXD because I thought I needed that constant f/2.8 for the neon lights in Shinjuku at night. Tbh, by the third day of walking 20k steps, my neck was killing me. It is a fantastic lens, super sharp and the stabilization is solid, but it makes the a6700 feel pretty front-heavy after a few hours. Next time I went out, I grabbed the Sigma 18-50mm f/2.8 DC DN Contemporary and it was a game changer for travel. You lose some reach, yeah, but it basically fits in a jacket pocket. If you really need the zoom range and plan to shoot mostly during the day, the Sony E 18-135mm f/3.5-5.6 OSS is a decent option, though you'll definitely struggle once the sun goes down. If I were you, I'd go Sigma and just crop in a bit if you need more reach. It keeps the whole setup light and fun for long days of walking.