All of these below are translated by translation software and may contain some wording and grammatical errors (as I am from China, my English is not very good and the Chinese expressions are different from the English.)
BUG.
When you are attached to another Mob, the berries do not do damage to you

Also, can you add a "real mode" to this mod?
The details are as follows.

1. in "real mode" you move at the same speed as the creature you possessed in vanilla (see: possessed zombies now move faster than other zombies)
2. In "real mode" you don't get the same effects as the mod (e.g. when you die, you don't get the "Attrition" effect)
3. In "real mode" the rule "requiem:possessionDetection" is dissabled by default, because in vanilla, zombies and other creatures don't attack their own kind for no reason
4. When you are possessed by a zombie or other hostile mob and you attack another hostile mob, only the mob you attacked will attack you back (e.g. if I am possessed by a zombie and I attack another zombie, the other zombies around me will not attack me while that zombie is attacking me)
   Your digging speed is also related to the possessed creature, for example if you possess a zombie, your digging speed will be as slow as when the zombie breaks down the door, even if you change to a higher level of tool or enchantment efficiency (this may be optional or not)
5. you will have night vision
6. pandemonium's Banishment soul will still work
   (All of the above rules are in real mode, but can of course be turned on separately with gamerule)

There are so few mods that play as a Mob in Minecraft that I would like to add the mod I'm talking about or make it a gamerule.

Translated with www.DeepL.com/Translator (free version)

Your "real mode" suggestion should be feasible right now with a recurring couple commands :

1. The slight speed bonus is intended to keep the possession experience enjoyable since regular zombies are excruciatingly slow, but you can add a slowness modifier with /attribute on every possessed mob if you so wish.
2. You can use /effect clear @e requiem:attrition to disable attrition
3. Feel free to change that gamerule, it's made for that 😄
4. In vanilla, if a zombie attacks another, all zombies will attack it. Requiem does not mess with mob interactions when possessionDetection is disabled. I guess there is no good way to get a constant mining speed like you want, but it seems a bit out of scope for this mod. You can always lower every possessed entity's attack speed though.
5. Again you can just give night vision to every possessed mob (/effect give @e["requiem:possessor"=!] night_vision)

bug反馈和feature request应该分开提交。
“附身”这个词对应使用“possess”而不是“attach”。
你想要的所谓真实模式可以通过更改对应文件的方式来达到，官方wiki上有教程。
关闭灵魂损耗效果可用命令方块。