I went through this same struggle before my last trip to Osaka. Honestly, I ended up being very satisfied with the Panasonic Lumix G Vario 14-140mm f3.5-5.6 II ASPH POWER OIS. Its such a reliable choice because you never have to risk your sensor getting dusty or wet by swapping lenses in the middle of a busy street. I was really happy with how well the dual stabilization works on the G9, even at the long end. The image quality is surprisingly solid for a super-zoom, and it only weighs about 265 grams so I had no complaints about my neck after a full day of walking. You can find them for around $450 used which is a total steal for your budget. Its the safest bet for a one-lens setup if you want to be ready for anything from close-up street food to far-off temple details. Let me know if you need more tips on that setup!

To add to the point above: if the kit lens was hurting your neck, you really need to look at the weight distribution. The G9 is a chunky body, so putting a tiny lens on it can actually feel a bit unbalanced, but its great for discrete shooting. I saw this earlier and wanted to weigh in because I have spent years lugging gear through Kyoto and honestly, the Panasonic Leica DG Summilux 15mm f/1.7 ASPH is a masterpiece for that specific street vibe. Pros: Insane low light performance for those neon alleys, tiny, and has a physical aperture ring that feels great. Cons: You will definitely feel limited at temples where you can't get close to the architecture or garden details. Another solid choice is the Panasonic Lumix G X Vario 12-35mm f/2.8 II ASPH POWER OIS. Pros: Constant f2.8 is way better than the kit for bokeh and night stuff, and it is fully weather sealed for those rainy Tokyo days. Cons: Still has some weight, though it is shorter and more compact than your current 12-60. In my experience, if you are worried about sharpness, check out the charts on the OpticalLimits website. They show that the 15mm is significantly sharper in the corners than the zooms. If you go prime, maybe just use your phone for those few times you need a long zoom shot. TL;DR: Grab the 15mm f1.7 if you want to fall in love with photography again and dont mind zooming with your feet. It makes the G9 feel like a totally different, much lighter camera.

If your neck is already giving you trouble, you might want to consider the Olympus M.Zuiko Digital ED 12-45mm f4.0 PRO. It is significantly lighter than the f2.8 alternatives at only 254g and is fully weather-sealed, which solves your rain concern without the bulk of a superzoom. Be careful with the f4 aperture for night shots in Shinjuku though; you will definitely be bumping your ISO. The G9 IBIS is solid, but you cant beat physics when the light drops. For a budget setup, I would suggest looking at the Panasonic Lumix G 20mm f1.7 II ASPH. You can usually find them for under $250 used. It is a pancake lens, so the weight is negligible. Just a warning: the autofocus motor is an older design and notably slower than your kit lens. It might hunt a bit in low light, which can be frustrating if you are trying to catch a quick moment. It is also a 40mm equivalent, which I find a bit tight for some of the narrower shrines, so just make sure you are okay with that field of view if you go with a single prime.

TIL! Thanks for sharing

If the neck pain is already hitting with the kit lens, you gotta be careful because the G9 is basically a brick at 658g. Slapping a tiny lens on it might make it feel totally front-light and weirdly balanced. Since you are worried about rain and want that pro look without the bulk of the f2.8 constant zooms, I would suggest looking at the Panasonic Leica DG Vario-Elmarit 8-18mm f2.8-4.0 ASPH. A few technical bits for you:

• It is only 315g, which cuts down your total rig weight significantly.
• It is fully weather-sealed with a solid build, so you wont have to baby it when the weather turns.
• That 8mm wide angle is honestly a cheat code for those tight Tokyo alleys and massive temples. Make sure to watch out for lens flare when you are shooting directly into street lights at night, tho. The G9 IBIS can handle the f4 at the long end if you trust your hands... but definitely keep an eye on that ISO if things get real dark.