I'm confused on what you are saying.

what do you want?

This part is already in the game.

As for the trade system thing, I think it sounds like a good idea, but practically it would be really difficult and trading with anyone you don't share a border with would be practically impossible.

That would in fact be a way to bypass the SBDE restriction, which I know it is not desirable. By that method you could have 5 tacs Lvl4 and 5 tacs Lvl3 in a stack that would operate with 100% SBDE.

Same for all other units as well.

That is a brilliant idea. I may just have to try that with an ally sometime.

At least with the trading with the ally the units will be forever stuck in that level.

With this proposal there is simply no drawback (at least till they fix this loophole, as I think they have in mind).

I am absolutely confused on the topic as your level of grammar is beyond terrible.
The only part i got is the trade system, i dont agree with it, because those are trading civilian ships they arent protected by arms and a simple Interceptor could make a massacre in the ocean.

The War Thunder tech tree is designed for an entirely different game. Some tanks and planes have very little and very specific differences from each other, things like faster turn speed of planes and faster rotation of a tank turret. Keep in mind that each "tank" or "plane" you make in call of war is actually a squadron or large number of unit working together. An infantry unit is 1000 soldiers for example, so tiny upgrades like these are not going to make any difference in the grand scheme of things.

Though upgrading your units is very important as it gives you an extra advantage against an enemy that doesn't. For example if your artillery has an update it gets a bigger range. This means that you can attack without being counter attacked because the enemy has a smaller range. Same goes for tactical bombers, they get a bigger range as well.

I like the idea of many different units with slightly different abilities, it would make the game more dynamic and possibly add a non gold moneymaker for the game, but it may put the advantage in some country's hands and may overcomplicate the game.

It would indeed be much better if your existing units didn't automatically upgrade when you researched a new level. You should have to pay resources if you want to upgrade an existing unit (maybe additionally add an "upgrade all" button that costs more if you already have more units of that type and/or these are several levels behind your state of research).

This would make the now common strategy of hyper-focusing on a few unit types and researching these to the limit while fully neglecting others less rewarding (although the research balancing update has already very much improved the situation a lot in this respect).
Also would be more realistic - of course old machines didn't turn into new ones by magic the moment a nation achieves new technology know-how.

But that's not an easy change:
* Firstly, as atreas1 already remarked, SBDE calculation would have to modified. Every unit of the same type in a stack would have to count for SBDE, regardless of its level.

* Secondly, the display of a stack that contains units of same type but different level would have to be adapted. For example like this - example for a stack with one infantry level 2 and one infantry level 4 (please excuse German texts):

(transparency in the stripes for unit levels that some, but not all contained units of that type have reached)

And unit detail view (places with changes in orange boxes):

Thirdly, an upgrade button would have to be added and calculation how much resources it would cost for the selected stack.

After trying out Supremacy1, I have to add that an option to upgrade existing units isn't mandatory.
Would be nice, but not mandatory.