In my experience, finding the Sony Alpha 7 IV 33MP Full-Frame Body under $1800 is rare. I've tried many setups, and while the a7 IV has that amazing full-frame depth, the Sony Alpha 6700 26MP APS-C Body is much lighter for walking around Osaka. The a6700 fits your budget perfectly and honestly, the autofocus is just as snappy for travel shots.

Regarding what #2 said about "Had a moment to think about this more while looking at my own gear"

• I think that's the smartest way to look at it. If you're hitting Japan in two weeks, you don't just need a camera body, you need a functional system. The technical reality is that full frame glass is significantly more expensive and heavier than APS-C stuff. If you spend $1800 on a body, you're basically broke before you even buy a lens. I'd honestly suggest looking at the Sony Alpha 7 III 24.2MP Full-Frame Mirrorless Camera instead of the IV. It still delivers 15 stops of dynamic range and has the dual SD slots which are crucial for backing up your once-in-a-lifetime trip photos while you're on the train. More importantly, it uses the Sony NP-FZ100 Rechargeable Lithium-Ion Battery, so you wont be hunting for chargers every two hours. Since your budget is firm, grabbing a used a7 III leaves you enough room to pick up a solid zoom like the Tamron 28-75mm f/2.8 Di III VXD G2 for Sony E. Having a constant f/2.8 aperture is gonna be way more helpful for those night shots in Shinjuku than the extra megapixels on the a7 IV would be. Just makes more sense technically and financially for a travel rig. If you need help comparing the sensor read-out speeds or buffer depths, just lemme know.

Had a moment to think about this more while looking at my own gear. Im really satisfied with sticking to slightly older, proven models because they just work. I took a similar trip and had zero complaints with the reliability...

• Sony Alpha 7C 24.2MP Full-Frame Body is very compact for travel.
• Sony FE 24-105mm f/4 G OSS Lens is a reliable all-rounder. Its a safer bet for your budget tho.