[OnePlus 3 & 3T Cross Device Development] mad-murdock:
(regarding all your posts)

wiping /data is different from wiping any other partition. usually and traditionally wiping means formating it. but since google moved the sdcard to the /data partition in android 4.something(?) with its nexus line, wiping data means only deleting all files without /data/media which contains the now-virtual sdcard.

so wiping on twrp->wipe results in said deletion with excluding sdcard. twrp->wipe->advanced_wipe does a real format of the whole partition with removing your encryption key. twrp->wipe->advanced_wipe->change/repair_filesystem->change_filesystem (something like that - hard to memorize the exact path) does format your whole partition to the selected filesystem without removing the encryption key.

if you really want to start twrp faster and switch to ext4, do a nandroid backup, backup all internal_storage to a pc (use the fast method from my signature), do the change_filesystem to ext4, restore all your data from pc and restore your nandroid backup. takes a bit more than an hour on average. ext4 has some more advantages, mainly stability if you like testing different roms.

I already answered your PM. Still, for public record:

Your /data partition is encrypted, since you are on stock OOS. If you are not asked for a PIN code on boot, your encryption password, which is automatically use and which you must enter into TWRP is "default_password" without the ". Entering that, you can access /sdcard and flash all the files you got there. Make a nandroid backup before doing so, in case anything goes wrong (broken zip, wrong version), you are safe then.

Magisk, SuperSU, disabler-zip all do the same thing: Unpack the kernel's ramdisk, change the fstab -which contains information of what to mount where with which options- and remove the two mount options, one for force encryption, one for dm-verity check. If there is no such entry (left), nothing is removed. So you can - in theory - flash all 3 of those zips, and your kernel is fine.

TWRP used the crypto libs to access your encrypted partition from the /system partition. The reason is, there are various incompatible versions. Maybe you remember the time, when there were different TWRP versions for either OOS 3.x or OOS 4.x. This is resolved by TWRP using the crypto libs from the currently installed system. Works like a charm and solves the problem.

Unfortunately your habit screwed you. Rebooting after wiping system does mean, TWRP cannot find the crypto libs and so not decrypt your /data partition. I suggest you drop that habit asap.

How to fix it? Without mounting /data, flash the last used rom (see below), reboot recovery and now /data decrypts fine after entering the password.

How to flash without accessing data?

Either you "adb push <file> /tmp" the file to the /tmp directory and flash from there. The /tmp directory is stored in memory. Having 6GB of RAM and roms being up to 1.5GB leave enough room for the operation. Also you dont need to delete anything afterwards.

Or you chose "ADB sideload" in twrp->advanced options and on your PC you do "adb sideload <filename>". The pc sends the file over to recovery (afaik stored in /tmp - lol) and flashes it afterwards.

Both options are equally good.