If you want to decrypt data on stock EMUI, you need to format data on TWRP - this operation will delete all your Internal Storage ( like your music, photo, configurations ... - all what was saved on Internal Storage) : External Storage ( microSD) will not be affected.
But, when you'll boot on EMUI ( after data was decrypted ) you need to flash also a custom kernel without "forcing data encrypted" - the stock EMUI kernel will encrypt data back after you boot from TWRP ( with data decrypted ) , because into stock kernel there is a flag on fstab.hi3660 which will force data encryption automatically ! I named this flag here : "forcing data encrypted" - for everyone to understand more easily
So, you need a custom kernel with fstab.hi3660 modified in order to boot successfully on stock EMUI with data decrypted .
Warning !
From device to device ( referring to the older Honor 8 & the newer Honor 8 Pro ) to this solution may be required also others modifications as well.
Regards,
EDIT :
If you read carefully what is written in the OP on the both sections :
Going back from decrypted -> encrypted? &
Want to decrypt? No problem. - you'll already have the answer to your question

....
but I've detailed a little more - for new users who are not yet familiar with Huawei / Honor devices !