I agree with fellow architect, jvlee that the answer is yes, Sketchup is great for this. Especially since you say
“optimalising anything is almost always a wasteful thing to do”
I am not sure exactly how you were going about it with Excel, but in my relatively limited experience spreadsheets are strictly mathematical and would provide you with an optimized solution limited to your input parameters. Assuming you are keeping it simple as per your requirements, the formula may not take into account practicality, priority, material properties, etc.
(For example: the volume of 3 fishing poles may be less than the volume of your backpack but it would not necessarily be a “good fit”, or it may tell you that they won’t fit due to the one long dimension but you know you could leave them sticking out the top and still have room in the backpack.)
Sketchup however would give you the ability to manipulate volumetrically accurate 3D models of the vessel and all contents. Yes you could move items onto each other, and into each other. You could arrange contents as you would in real life by simply dragging the objects into the volume, rotate objects, make outer vessel transparent, orbit, zoom, etc. quickly seeing if it is possible to fit everything comfortably and practically, leaving complexities to be decided on your own.
(For example: you know that a tshirt can be folded up very small, or wrapped around a fragile object to add extra protection, or even stuffed inside a saucepan, which would also be possible to do in sketchup)
This task would of course be even simpler if all manufacturers provided 3D models of their products, which some of them do. But you could very easily generate the basic volumes by researching the dimensions and images of the products without purchasing them first.
When I first downloaded sketchup I started modeling many of my possessions just because it is fun. Never thought to use the models in such a constructive way, great idea!