I want to experience what the new updates have to offer, but with vanilla's mob AI and item balance, combat and survival is just not engaging. I used to have mods like Epic Siege Mod which added great depth to the interactions I had with hostile mobs, but that mod like many mods I like are stuck in older versions like 1.12 because Mojang refuses to give Java an official modding api with proper version porting options. So I am looking for an alternative mod that works 1.13-1.16 and makes the game difficult primarily by giving existing mobs new behaviors. I am not looking for a mod that adds 100 bosses and I am not looking for a mod that doubles all hostile mob damage by 2. I want every fight I have with hostile mobs have more depth than hold down shield and crit with axe.
It could be anything from giving the vanilla monsters new attacks, like giving spiders a poison spit projectile attack. It could be AI improvements like allowing mobs to jump over gaps or flank the player properly. Heck, it could even be something as simple as nerfing the player by making shields less op. I think I've given enough examples for people to have an idea of the type of mod I'm looking for.
Another possible workaround is if anyone has a 1.12 mod that backports features from the new updates into older versions, that would work to.
As someone that looks at 1.13+ mods across Forge & Fabric I don't think I have come across many tougher Mob AI mods (that and they aren't something I seek out) but there is mods like In Control that might allow tweaking of mob spawns and other aspects, or AI Improvements which might be more what your after by the sounds of it https://www.curseforge.com/minecraft/mc-mods/ai-improvements, maybe even Champions might have some benefits https://www.curseforge.com/minecraft/mc-mods/champions, or there is Infernal Mobs, Lycanites and more that adds more mobs. But I assume your after tweaking of Vanilla mobs.
I document mods daily from 1.13 to 1.16.2 but I don't remember a lot of them and I've not reviewed any mob AI mods so I'd say look around and see what's out there as for 1.13 there isn't likely but 1.14, 1.15 and 1.16 there should be or some have been ported to 1.14+ from 1.12.2 with enough aspects to them to make the game as challenging as you want it. Or going with 1.14+ Raids that Vanilla added and playing on a harder difficulty, the Enchant With Mob mod https://www.curseforge.com/minecraft/mc-mods/enchant-with-mob that adds the Minecraft Dungeons mob that provides shields for mobs for example. I can only guess though as it's not really my area of modding I'm familiar with.