This is the stuff I used to mod my game. Its about 3 mods to get rid of freezing and upograde graphics

Ultimate Asi Loader (https://github.com/ThirteenAG/Ultimate-ASI-Loader/releases/download/v4.68/Ultimate-ASI-Loader.zip)
Eagle Patch (https://github.com/Sergeanur/EaglePatch)
XOutput (https://github.com/csutorasa/XOutput)
Community Remaster (https://www.nexusmods.com/assassinscreed/mods/32)

Freezing is caused by AC1 trying to connect to a Ubisoft server that doesnt exist anymore (thats why disconnecting your ethernet/wifi helps). This is solved by going to C:\Windows\System32\drivers\etc, opening host in notepad, and add 203.132.26.155 127.0.0.1 to its own line.

I use two graphical mods, one is EaglePatch which adds multisampling among some other things (it does get rid of any Dinput support) and the community remaster of the game.
to start off you will need to install Ultimate Asi Loader (see above for link.) First, go to your AC1 folder, then open Ultimate-ASI-Loader.zip and drag d8input.dll into your AC1 folder (should be the same folder with your exe). Afterwards, create a new folder called scripts and then open EaglePatchAC1.rar (you can use winrar or 7zip). Take EaglePatchAC1.ini & .asi and put them into the scripts folder.
The community remaster is a collection of different mods for ac1 all packed neatly in one installer. This includes better textures, better lighting, discord rich presense, and some others. To install this you just need to run the installer downloaded from nexus.

The way I launch the game is by adding {"D:\SteamLibrary\steamapps\common\Assassins Creed\Assassins Creed Remastered Launcher.exe" %command%} to the Properties tab on Steam (replace it with the path of your file, its probably on c drive). If not you can rename it to AssassinsCreed_Game.exe or create a shortcut
If you want dinput but have installed Eagle Patch, you will need to use a mapper like XOutput to be a middleman and handle those inputs for you.
sorry :(
https://www.reddit.com/r/assassinscreed/comments/y0mfsj/assassins_creed_1_remastered_the_definitive/
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=3156091793
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=3046189346