Look, I totally feel your pain. When I first got my a6000 series for a trip to Osaka, I was honestly miserable. Unfortunately, the out-of-the-box experience for the a6400 is not as good as expected, especially with that tiny NP-FW50 battery. It is frustrating that such an expensive piece of kit feels like it is gonna slip out of your hands every two seconds. I had issues with the battery dying mid-shrine tour, which was just the worst. Here is what I would grab to save your sanity without blowing that 200 bucks:

• Get a grip or a cage immediately. The camera is way too small for human hands. I use the SmallRig Cage for Sony A6400 2310 and it makes the camera actually feel solid and easier to hold.
• Ditch the sandpaper neck strap. The Peak Design Slide Lite Camera Strap Black is a lifesaver. Its smooth, fast to adjust, and wont chafe your neck while you are walking 20k steps a day in Tokyo.
• You need more power. Sony batteries are overpriced and dont last. Grab a Wasabi Power Battery 2-Pack and Dual Charger for NP-FW50 so you always have spares in your bag.
• If you dont have a fast card, the camera will lag while writing files. I have had issues with cheap ones failing, so I only trust the SanDisk 128GB Extreme PRO SDXC UHS-I Card. Seriously, dont let the stock setup ruin your trip. Fixing these things makes it a totally different camera. Tokyo is gonna be incredible, just gotta get the gear sorted first...

To add to the point above: the NP-FW50 batteries only have a 1020mAh capacity, so managing your power draw is vital. Beyond that, you should be careful with your media selection and physical handling.

• Media: I would suggest SanDisk Extreme Pro 128GB SDXC UHS-I V30 200MB/s. Sony's XAVC S codec requires a Class 10, U3 rating for consistent 100Mbps bitrates. Lower-spec cards can cause write errors or buffer clearing delays during fast bursts.
• Safety: Since the body is slippery, make sure to look at the Peak Design Cuff Camera Wrist Strap Charcoal. It has a self-locking feature that prevents the camera from hitting the pavement if it slips from your hand in a crowd.
• Screen: Sony's LCD displays are quite prone to delamination. I'd highly suggest an ULBTER Tempered Glass Screen Protector for Sony a6400 to maintain the 9H hardness rating against scratches. Be methodical with these and you'll avoid technical failures in Tokyo.

totally agree, the power management on these is wild! For maximum reliability, definitely pick up the Anker 737 Power Bank PowerCore 24K 140W. I love it because the 86.4Wh capacity is flight-safe and provides amazing peace of mind. It handles the 5V input perfectly, basically giving you infinite runtime while walking between shrines! Its a total lifesaver for safety since you wont ever get stranded with a dead camera.

In my experience, your handling issues are best mitigated with an L-bracket. Check the DPReview guides for setup ideas.

• SmallRig L-Bracket for Sony A6400 2331
• LensPen NLP-1 Cleaning Tool