[Nexus 6P Q&A, Help & Troubleshooting] Citizen_Snips:
No offense intended, but this always irks me quite a bit.

Absolutely DO NOT use toolkits for anything at all, ever. They're not easier, they're not faster. It's just doing fastboot / adb with a UI that has buttons you can click instead of typing in fastboot.

This. This is the exact reason you shouldn't use toolkits. Especially if you don't know the manual way. It borks something up and now you need to wait possibly hours to get a working phone again when the solution would only take about 15 seconds to fix.

You need to fully familiarize yourself with this guide here;

https://xdaforums.com/nexus-6p/general/guides-how-to-guides-beginners-t3206928

ADB/Fastboot is absolutely not "hit or miss." If you have trouble with it then you need to make sure that your drivers are properly installed and you're entering commands correctly. Also make sure to ALWAYS keep your ADB & fastboot drivers up to date via the Android SDK. All of this is covered in the link above.

As far as your bootloader issue. You cannot return to a locked bootloader with anything other than 100% complete unadulterated stock factory images. No TWRP, no root, no Xposed, no Magisk, no build.prop edits etc., ...Nothing.

Use fastboot to flash your factory images you downloaded. You can use the flash all bat file or individually flash each separate. Again it's all covered in the link I posted above.

Also, what do you mean you couldn't wipe/reset from stock recovery? When you flash stock recovery and boot to it, you will see an Android on its back with an exclamation mark in a red triangle. You may or may not see text that reads "No Command". Either way, from that point hold down the power button and then press and release volume up. You will boot to stock recovery where you can wipe your data.

Not for nothing, but as mentioned above, people have hard bricked their 6P from locking bootloader's on combos besides stock. You got extremely lucky you didn't as well. Please take some time to read and fully understand what you're doing with your ~600$ device.

This is also, for the most part, pretty standard stuff as far as flashing / modding goes. At least since 2010 when I got into it. If you want to know the exact string of fastboot commands please read and use the guide I linked. It teaches you literally everything. There's nothing it doesn't cover and once you get used to it you will never use a toolkit again.

Hope I didn't come off rude, but basically since 2010 when I got into this across all my devices I try to help out in the Q+A sections. I've probably participated in hundreds and hundreds of "help, I used a toolkit and my phone is bricked and I have no idea how to fix it, I just wanted cool stuff on my phone" threads. They're absolutely 100% not worth the trouble and headaches from them. Especially if you have no idea what they're doing on the back end. Know Fastboot and ADB like the back of your hand and just prefer to use them? That's a different story, if something fails then you fix it yourself in 10 seconds. But for the love of God, please please please please please stop using toolkits if you don't know how to fix issues on your own. You will eventually hard brick. I've seen it happen dozens of times. Thread turns into a circlej*rk about whether toolkits are bad or not, OP hard bricks their phone and commits fraud, which is illegal by trying to RMA it. Just don't use them. It's simple enough without them. Why reinvent the wheel when it works just fine?