Hi!
I wanted to use optifine on this mod pack, so as it's said, i've deleted magnesium file. Unfortunately, after few minutes of playing game crush. This is what the information in minecraft launcher says

Without optifine file game runs well

You need to remove the following mods:

• SimpleDiscordRichPresence-1.4.0-build.3+mc1.16.5.jar
• magnesium-1.5.jar
• screenshot-to-clipboard-1.0.7-forge.jar

After that, you should be good to go. I had the same issue when experimenting with shaders.