Suspension upgrade advice

I would also agree with others that the Jimny woudn't be my choice of 4x4 as a long distance overlander . It can be done and there are plenty of examples but you are limited as to what you can do and every mod you do to the Jimny will impact its road handling in someway. My opinion is a suspension upgrade can improve the ride quality a bit but at the end of the day its still a short wheel base car that feels every pot hole and will be thrown about. There's nothing you can do about the windage and roof racks, tents, suspension lifts will only make it worse in the wind. I wouldn't see the point in a difflock in a Jimny unless your really offroading in extreme terrain which I wouldn't think you would be doing in any sort of overlander vehicle anyway. The Jimny is kind of in a league of its on compared to the other vehicles you've mentioned and will handle very differently. You can only try each car and see which one you like and get on with the best to makw your decision. 

If going down the Weangler route it would be hard not to go for the bigger unlimited because of this mode:  ursaminorcampers.com/products/jl-j-30-camper-2018-2023

By far the slickest sleeping option.

But here is the problem - there is always something up from everything.

I don’t have a fixed budget, but also don’t want to be irresponsible. For example by the time such a W is setup I could get 2 Jimnys.
Running costs probably higher too.

As for generations I was talking about the 4th Jimny and also JL W. Now if going for JK things do get much more competitive.
 

Have to add that I am in a small country with extremely limited local offers and would most probably have to import.

Even within EU CO2 taxes (by whatever name) are so high for stronger diesels that Wranglers, depending on trim and engine, get a 5.000-10.000 tax to factor in on the price.

Except for the Hybrid that is almost nothing.

Ah I had assumed you were talking about the gen3. I have both and believe me distance in a gen 4 is a much less appealing prospect. Yes they ride softer, have cruise control and get more mpg but they are noticeably louder inside which some might say turn up the stereo but that just makes more tiring. I have done all day 10 hour driving in both of mine and in the new one I feel like I need a couple of pints in the pub to recover whereas in the older one I just want to keep going and see what is round the next corner.

Also whilst there is more absolute volume inside the new one there is less in the way of convenient little pockets to put things in.

We are all different and there are plenty of people who can essentially live out of the back of their jimny and do it regularly but if it were me I'd use my short land cruiser to do a multiple day/week style trip.

Oh and one other thing the fuel tank on a jimny is 40 litre or approximately 200 to 250 miles at most, significantly less when modified.

Just for context I know it sounds like I am anti modified jimny, I'm not, in fact my older one is bit by little turning into a sleeper, so I understand the urge to put them to a use they weren't intended for. But making one go round corners is less of a task than making one fit to live out of.

Also whilst I think about it the total payload of any jimny is only around 320kg ish and includes a driver in that and in the lcv van the rear payload is stated as 150kg. This and the roof limit of 30kg come from suzuki and while both can be exceeded in the event of a collision you can expect the authorities and insurance to play hard ball if you have gone over.

I done alot of overlanding with my jimny. Probably 30-40 nights a year the last few years.
Not for very long trips or expeditions though, but a few trips of 2500km.
My car is lifted 2". Bigger tires can take low pressure for slower driving making it more comfy on Mountain roads. 
Worn out a set of 235 bfg MT, now an old set of 215 AT while I make up my mind what New tires to buy. 
I have a light weight trailer. Does not bother my driving much and can follow a bit offroad too, at least everything legal here.
I do not have a rtt. I often travel with friends with tents and just the hassle of finding camps without any wind. Many have rtt on their jimny but they do not follow my speed in curves. mpg is more or less the same with a rtt as my trailer. 
An awning to me is also important and if you add that weight to the tent on your car you're in trouble.
I have just the awning on my roof.