First, there are some misconceptions about knives:
- There is no such thing as an all-purpose knife. A knife's blade shape, steel, length, weight, type and angle of edge all determine what a knife is best at. While it surely will do other cutting tasks, it will never excel there. Take a BRKT Bravo 1, for example. It is an excellent knife for heavy duty tasks, and in a survival situation, I'd bet my life on it. But it s***s big time for everything else. Food prepping is a plain horror, and the rather light campsite tasks are no fun, either. On the other hand, you would not want to try to split wood with your Victorinox Classic, would you?
- A knife does not have to be expensive to be good. A Mora Companion Heavy Duty surely will never win a design price (except for "The Ugliest Knife built. Ever."), but it is one of the sturdiest, most easy to sharpen and dependable knives I have ever had the pleasure to put my hands on.
- Last but not least: a knife does not have to be big to be useful. While some tasks become easier using a big knife, almost everything can be done using a small knife.
Ok, now your tasks are mainly light cutting tasks, with an implied hiking. So you need a knife suitable for light to medium tasks. As for hikes, I'd always imply the use as a survival knife. I'd suggest a neck knife, as it comfortable to wear, easy to conceal (running around with a visible bald may be unacceptable in some areas – and even illegal) and quite easy to handle.
Since you want to cut fish and plants, which I'd roughly classify as food prepping calls for a flat ground edge.
A drop point or clip point shape is ideal for extra control during light to medium tasks.
As for the steel, the general question is wether you want stainless or carbon steel (setting aside all shades between those two for now). As your question implies that you don't want to put much, if any, work into maintenance during the travel, I'd suggest a stainless steel.
Since you are willing to pay some money, I suggest a knife you will be happy with even when you "get into it" and buy other knifes, for more specialized tasks.
Why vote goes for the ESEE Izula in the stainless 440C steel with a paracord wrap as handle. Not an awesome steel, but a good workhorse. The edge retention is not great, but it will retain a good working sharpness during all but the longest trips. It is lightweight, sturdy, suitable for your tasks, legal in many legislations and not too expensive. Though even when you will buy other knifes, you will come back to your ol' trusty Izula.
As a backup, I suggest a Victorinox EvoGrip S18. It has all the tools you need outdoors and then some. It even offers some of the highly underestimated tools for personal hygiene. If you can get your hands on the older models built by Wenger, I'd suggest taking that instead. The slide in tools are built a bit nicer, imho.
Some side notes: I am not a fan of integrating survival tools into a knife. Such a Frankenknife will always be good in one part and sub-par in the other. When it comes to survival equipment, you want the best of breed for every single item. I am no fan of serrated blades, either, not even partially serrated. They can do nothing a fine edge blade can't do, but you simply can't do a clean cut with a serrated blade.