Must Have:
Save confirmation dialog skipper
-Skips the annoying and unnecessary save confirmation dialogue

Alias Isolation
-the original game suffers from severe aliasing, which this mod fixes with the addition of proper temporal anti aliasing
-even though the linked mod version was created for the epic version of the game, it also works fine in the GOG version
-requirements: "Anti-aliasing must be set to SMAA T1x. Chromatic Aberration must be disabled. Motion blur must be enabled."
-usage: "To disable the mod at runtime, hit "Ctrl+Delete". To re-enable it, hit "Ctrl+Insert"."
-the setup instructions on github are a little bit unclear, therefore I advise you to do it this way:
-- make a folder "AliasIsolation" inside the game directory and put all the mod files into it
-- create a batch file called "StartAI.cmd" inside the game directory with the following content:

@echo off
start /b "" AI.exe
start /b /wait "" /d "AliasIsolation" AliasIsolation\aliasIsolationInjector.exe .\AI.exe
exit

-- start the game via the batch "StartAI.cmd"
-You will only see if it works after starting/continuing a game, since the logo doesn't show up at the title screen (at least for me)

Recommendations:
No lensflare mod
-disable the rather ugly lensflare effect when looking into bright light sources

Improved Alien
-changes the behavior of the alien to make it a little bit less frustrating and more predictable

Config Tweaks:
Better LOD distance
-open "DATA\ENGINE_SETTINGS.XML" and set "Ultra" setting of the "Level of Detail" group to "2.0":

<Setting name="Level of Detail">
<Quality name="Ultra" float="2.0" precedence="4"/>

I would add the mod to hack the alien away.

Yeah, AliasIsolation works if following your solution - just check my simple comparison for the nasty jaggies on that box - incredible!

But do you know how to remove the Chromatic Abberation it turns on by default?
I always hate that effect, even if it's better than the horrible aliasing, and in this cinematic game it might have some purpose, but I would rather have it off anyway...

I've add some more mods to my op. Strange, I don't have this problem. No CA at all with this mod.
Are you sure that you didn't accidently reactivated CA in the video options?

Yeah I'm sure.
And I'm pretty sure it adds CA by default.
You can see it on the comparison-screen I posted (since those ugly jaggies was out towards the edges of the screen it also shows a bit of CA).
In my attempts to get this working earlier I started some GUI for it, which had two sliders and one of them was for CA and by default it was in the middle.
But I'm starting to think maybe the CA adds something to this particular game...I can live with it anyway :)

I think I have a solution for you. Open "AliasIsolation\data\shaders\chromaticAberration_ps.hlsl" and change the code under "PSOutput mainPS(in PSInput IN)" as follows:

PSOutput mainPS(in PSInput IN)
{
PSOutput OUT = (PSOutput)0;
OUT.param0 = float4(colorTex.SampleLevel(LinearSampler, IN.param1, 0 ), 1.0);
return OUT;
}

This should disable the CA effect completely.

Not sure if I want to remove it all, maybe just tone it down a little.
Maybe just change "float ca_amount = 0.018 * g_caAmount;" from 0.018 to 0.010 or something, but not sure if it's a good idea :D

EDIT: Yeah, setting this value to 0 eliminates CA totally - probably easiest way.
I have it on 0.010 now and think it's a good amount, giving just a tiny bit of CA :)

Here are some ideas for tweaks.
I tried the shadows one (higher res), and I get really sharp and nice shadows, but on other places they get more blocky instead and look really bad.
Like on survivor "Basement", just at the start, in front of the door you open to start the level.
The light and shadows coming through the windows look way more blocky and are creeping/flickering if I go up to 4096 instead of 2048.

Haven't tried the reflection tweaks, I don't know if they might be a bit too much, floor looking like a mirror, maybe wasn't like that in the old movies I don't remember...things was pretty dirty I think :D

https://www.pcinvasion.com/make-alien-isolation-look-amazing-4k8k-shadows-reflections-detail-and-more/

EDIT: Reading this in some guide - guess that's what I saw, and then I think I prefer lower res vanilla shadows instead...
"Increasing shadow map resolution introduces shadow flickering in some areas."

Missed one, the essential modding Tools of Matt Filler, culminating in the 2019 release of Open Cage, the collection of all his separate tools into one.

OpenCAGE file - Mod DB

Still being updated, at least it updated when I just started it after changing from Steam version, to new GOG release.
Note: I had to edit JSON settings file manually, or just delete old JSON settings file, so it asks for path to game, as on first use.

The instructions for Alias Isolation did not really work for me, giving a "Could not create process..." error when running the .cmd. However, I still got it to work by starting the game through aliasIsolationInjectorGui.exe.

Have you tried this updated fork?

https://github.com/RyanJGray/aliasIsolation

Finally, I managed to made it work. It's HDR. With Windows HDR on, everything gets blurry. A note on that page gave me the hint, so, thanks a lot.

"Windows Auto HDR or any other software which also uses API hooking may break the rendering"

Late to the party here but I recommend the "Unpredictable Alien Mod"

It won't rubber band as bad to the player. That means you could go for a long time and never see the alien, but the encounters you do have should be more memorable. It's not perfect, but it's damn good.

https://www.moddb.com/mods/unpredictable-alien-mod