Anything that is purely visual is fair game for mods. - reshade, textures, HD models and new guns (As long as they are balanced according to the original game) detract nothing from the overall experience.

Same with task mods like Arzi/IGI tasks, having more missions is great and they use the vanilla framework so it is serious.

The only mods I would avoid are mods that change the balance of anomaly, add new features/mechanics and add artificial difficulty to the game.

Such as adding EFT style features like body health system and Magasines reload/loadout. Extra mutant spawns etc,

There are plenty of Vanilla+ style modpacks that do this sort of thing. Then try stuff like the excellent Escape from Pripyat modpack later.