Still in search for a proper vision mechanic (When a new player joins)

For a screen of level N, there is no range reduction but you can only have up to N of them before the visibility gets one level worse. So the thick forest gives you a 2/6 chance to hit. Up to two of them can be in your line of sight and remain 2/6, a third lowers you to a 1/6. A second 1/6 (which might be three other 2/6's) makes it 0/6 and the shot becomes impossible.

So seven 6/6's becomes a 5/6. Over that kind of distance, even small problems accumulate.
Six 5/6's becomes a 4/6.
Five 4/6's becomes a 3/6.
Four 3/6's becomes a 2/6.
Three 2/6's becomes a 1/6.
Two 1/6's becomes a 0/6 (impossible).

The other idea is to sum up the screening points and do something with that. Assuming 6/6 for clear terrain the heavy forest's 2/6 becomes "4 screening points" for passing into or through that space. Maybe each N points reduces the range or lowers the hit probability. So shooting into an adjacent heavy forest has little or no penalty, but any combination of terrain can quickly ruin your day.

As a suggestion why not group terrain most wargames keep terrain simple so it's easier for players to understand how they work.

You could have say light cover that provides a small bonus to defence and then increment it based on how good the defence is. Of course if the defence blocks los that's simple enougth as they can't be shot at.

You could also have terrain affect accuracy so let's say a forest your harder to hit and you have a bonus to defence if you are hit. Hope these suggestions help.

Seems like you have an interesting game I hope to hear more about it.

I think I would need to see how your game works to fully understand your system as for advice I have the following.

You could have tthe forest effects be static and always provide the same bonus regardless of how many firsts there are having large forests just means you have more advantageous terrain to use.

As an example of a game stystems forest rules I'll use warhammer fantasy
While in forest missile attacks are at a - 1 penalty but the unit also can't get bonuses for being in a group as they have to spread out in the forest.
Also the forest slowed down units

If you keep your terrain effects set it won't cause Issues while it's illogical it's also simple and from experience simple is better rather than having to check countless charts and remember countless bonuses.

I used to go d&d 3.5 it's painful to run as players need to track so much information it can take them ten minutes to figure out their end modifier and damamage roll.

This also depends on your audience if it's a wide audience then defiantly keep it easy to remember if it's aimed at a more niche gamer market then I can see the advantage of having more complex rules.

While player feedback is valuable and should be considered sometimes as a designer it's best to go with what you feel works.

I'd most of your playtest have been fine with the old rules perhaps keep those rules as always keep testing it's hard to find the balance that works.