Apologies in advance if I offend, but… you’re not giving us a chance to give real World feedback (I’ve lived and travelled extensively in Scotland on a variety of photography expeditions) because you haven’t actually given us a clue on what you want to do with a camera, in those 17 days?

1. First question, why are you going to Scotland? Vacation (and what’s the theme & locations of the stay), Work, Visiting Family, Other? It would be useful to know how much time (for travel and shooting) & where in Scotland you can take time for your photography.

2. What do you want to shoot? What’s your favorite genre, that your “photographers eye” would be most naturally drawn to? Moody, wet & windswept Landscapes? Foxes and Red Deer photographic stalking in the subpolar western highlands? (BTW, both those locations will feel VERY subpolar, in October…). You mention Abandoned Castles; there’s some wonderful photo opportunities, but there’s a reason why all of them are abandoned… most of them bring a whole new meaning to the phrase “off the beaten track”.

3. Where? Obvious, but if you want to shoot the seabirds of Stac an Armin, then that’s going to take a lot more planning and prep than an impromptu walkabout & street photography shoot in the Royal Mile. Have you considered your travel needs to get to those locations, and how that will affect the equipment you need to bring?

4. Finally, what photographs are your highest priority? As you have a maximum of 17 potential days to shoot, and Scotland is a very large country, with nearly every variation of terrain you can find in NE Europe, you will probably have to prioritize your objectives. You also need to consider the weatherproofing of your camera equipment; Scotland is the windiest and the wettest country in Europe.

"Wide enough" indeed is a relative term, and different photographers legitimately have a different take on what is wide enough.

In the other part of my answer, I recommended the Sony 18-135mm--which is remarkably similar to the 18-200mm you are using, particularly at the wide end. So where is the disagreement? OP has a Sony a6400 and wants *one* light-weight lens suitable for a trip to Scotland. In my humble opinion, the Sony 18-135mm, while not perfect, comes close to meeting that objective.

The underlying issue is that many UHH users wildly mix full-frame and APS-C in their discussion. While the focal lenfth of course does not change, whether it is used on a APS-C or full-frame body, what the lenses do on the respective bodies changes a lot because of the crop factor involved.

Specifically here, some folks recommended a 24-105mm lens. This is a full-frame lens, which is a great one-and-done travel lens if strapped on a full-frame body, like my Sony A7 III. If OP were to use it on an APS-C body, like his Sony a6400, it would be 36mm (full-frame equivalent) at its widest, which in my humble opinion is not "wide enough" for an all-purpose travel lens. This is based on my significant travel experience--I am currently based in Nairobi and am on a two-month side trip to Ireland, Spain, Portugal, France (from where I write this), Germany, Switzerland, and Sweden. The 18-135mm would bring that to 27mm full-frame equivalent, which is not perfect but more workable than 36mm full-frame equivalent. As you say, it is what it is.