This is a very good post, I would like to add a quick insight on my thoughts though.
Personally I don’t think anyone nation is the ‘most suffering’ out of all, yes certain trees suffer more than others however it is more a collection of trees that are sort of left to die.
Imo France, UK, Italy and Japan all suffer pretty badly, and whilst yes Italy’s ground lineup is the worst at top tier, its air lineup is now very potent having an F-16, Mig-29, Gripen, Tornado x2, and a Harrier II (before the US i may add). and although originally the most forgotten nation recently the last year of updates has been very generous to them giving them both a HU ground and air tree (although air is very barebones right now) as well as several unique and interesting features/vehicles before many Major nations like F&F missiles such as the Spike.
Japan has also suffered quite a bit as its arguably the tree in the most dire need of a Sub-tree yet is going to be the second last minor nation (barring Israel) to get one, it also has some of the worst early ranks for Ground and whilst its air is good, a lot of its aircraft end up overtired because people don’t seem to have the Intelligence to know not to Turnfight Zeros.
The UK has suffered because its just sort of ignored, it has more than enough potential to be a Major nation on its own yet its players have been ignored, They ask for a light tank Line yet they get a SA subtree, they ask for more diverse top tier they get another Challenger (this one has a slightly better engine!) and so on. It also doesn’t help that Gaijin has left the Commonwealth addition in a weird sort of Limbo, meaning its both impossible for Britain to gain fully from the commonwealth yet its also almost impossible for a Seperate commonwealth Tree to be made.
France is just well, France… it’s the most Unique of the Minor nations with very little of its domestic Productions being found elsewhere however it just lacks in diversity because of this, and like Britain they have sort of been forgotten. Despite having many unique options to help it, it’s been left behind aside for a few select choices around the place, its Naval especially leaves a lot to be desired.
Israel is a very touchy subject right now, Gaijin cant afford to give them much focus due to current issues and just doesn’t have much Domestic options that aren’t variants of other nations vehicles, thus just cant be a unique tree without external help through a subtree, yet almost any nations it could use wouldn’t bring much to the table, the only realistic one being South Africa which is already taken by the UK.
In conclusion they all suffer differently and imo there’s not a clear way to determine the most suffering nation, had you asked a few years ago i would have said Italy (had I remembered it existed, it was that forgettable) yet now after all its had over the last year and a bit its quite hard to properly say whether it suffers much it just depends on what happens in the future. The biggest issue is the “minor nation syndrome” where people spend less money and time on minor nations so gaijin sees them as less profitable and gives them less, which means people spend even less time and money and leads to this runaway effect where gaijin prioritizes its biggest cash cows (which right now are the US, Russia, Germany and Sweden) over the ones nobody is playing