I understand all these dilemmas--I too want to keep my kit small while having the lenses I need. I have settled on the Sony 24-105mm f/4 as my main travel lens. I was on sabbatical for seven months earlier this year; I spent half of it in Europe (Ireland, Spain, Portugal, France, Germany, Switzerland) and the other half in Kenya. I had with me two bodies, Sony A7III (similar specs to your A7C) and A7RIII. Lenses: Sony 24-105mm G, Sony 100-400 GM, Zeiss Batis 18mm, Sony Zeiss 35mm f/2.8, Sony Zeiss 55mm.I used the 24-105 for 90% of the shots, the 100-400 for wildlife and for some landscape shots, the 18mm for when I needed wide (lots of church interiors), and the tiny 35mm for street photography, particularly in Nairobi. I never used the 55mm even though it is a great lens.

I could absolutely trust to get a great image with my Sony 24-105mm under just about any circumstance. I have used it for interior shots in churches and urban night shots. I shoot at f/4 and ISO 6400 which usually lets me shoot at 1/30 of a second, depending on available light. The image stabilization in both body and lens work great in tandem, so I get a good shot to about 1/15 of a second, handheld. On all my travels, shooting at f/4 never kept me from getting the shot, even in poor light. I understand that you have reservations about the lens because of the form factor, and I get that.

Clear Image Zoom (CIZ) works great with all lenses, but it also has limitations. It only works in JPEG. The focus system is not as nimble--you only get zone focusing. For landscape or city shots this generally is not a problem. I get very sharp images. In contrast to cropping, you do not lose pixels, so you still get all the details. I would test it before the trip so you know how to use it. I have it programmed to one of my buttons.

Prime lenses: I think that the Sony 20mm would be a good lens for interiors. It is wide enough to capture interior spaces and fast enough to get shots that are in focus even in dark spaces. The Sony/Zeiss 55mm is a superb lens. I do not shoot much at 55mm, but that is a matter of personal preference. (I never used it on my seven-month trip.) The same with 85mm: I have a nice 85mm prime lens that I never use. I personally think that this is not a useful focal lenght for travel. If you want to take a second prime lens I would take a 35mm lens, or perhaps a 28mm--again, this a personal choice. I use the tiny Sony/Zeiss 35mm f/2.8--it weighs only 120g. Sony also makes a good 35mm lens at f/1.8 that is quite small too. Sony also issued three small primes (24/40/50mm) specifically for the A7C. But the limitation with primes is that you invariably end up changing lenses, which is difficult to do when you are part of a group trip. Taking the Sony 28mm f/2 would be an alternative. I think that 28mm is a useful focal length for travel.

I go through all these questions before each trip. And each time I grab the Sony 24-105mm.The IQ is great across the board, it is extremely versatile, and I can make it work in difficult light situations. If I was on your trip, I would take this lens. I also would take the Sony 20mm (or my Batis 18mm). I also would take a small 28mm or 35mm prime--the Sony 28mm f/2 or 35mm f/1.8 being good options.