[OnePlus 8 Pro  ROMs, Kernels, Recoveries, & Other ] S4turno:
I'm currently trying another custom ROM, so I can't help with 1), but about 2):

- You can flash Magisk in Lineage's recovery (Recovery > ADB sideload and then "adb sideload /path/to/magisk.apk"), no need to patch the image.

- There's no need to reboot between each module installation (you can install all 3 at once and then reboot).

- Of the 3 modules you mentioned, only the Props Config can be interacted with. The other two are "install and forget".

- To fix the safetynet/apps that check for root, you need to select a "fingerprint" from the Props Config module and change a few things inside the Magisk app.

To avoid typing everything from scratch, I'll copy/paste a comment I left somewhere else. Skip the parts you already done (eg: installing the modules):

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0. Root:​

Download Magisk(the apk file is both an app and a flashable zip).

Flash the Magisk apk via the recovery (as if you're flashing a LineageOS build). If you're using the LineageOS recovery: adb sideload magisk-app-name-here.apk. If using TWRP, just select the install option and select the Magisk apk to install.

Start the phone, open the app, it will finish the installation.

1. Fixing the SafetyNet:​

Open the Magisk app and go to the settings menu.

Enable/check the "Zygisk" and "Enforce DenyList" options.

Reboot.

Now we'll download 2 Magisk modules (zip files):

Universal SafetyNet Fix

MagiskHide Props Config

Now let's install both modules:

Open the Magisk app and go to the "Modules" tab (at the bottom).

Select "Install from storage" and then select one of the zips/modules you download above.

Go back and install the second module/zip.

Reboot.

The setup is almost done. You have to select a "verified fingerprint", one that was "approved" by Google:

Using a terminal app (eg: Termux from F-Droid), type su and then press enter. Magisk should ask you if you grant Termux root access. Grant it.

Type props and then click enter.

Type 1 (and enter) to edit the fingerprint and then f to select a fingerprint. Just type the number for your brand, then the model, then the Android version. Then type the option to reboot.

[ Note 1: This functionality is added by the MagiskHide Props we installed above. Internet is needed for it to download the list of "fingerprints". If your phone isn't in the list or if the Android version doesn't go up to the one used by LineageOS, select the closest one. ]

[ Note 2: Let's say that you have a OnePlus 9 and select the Xiaomi 11 fingerprint. For Google, you have a Xiaomi 11. When you login to your Google account, it will say that you logged in from a Xioami 11, not a OnePlus 9. Doesn't matter much in practice, just saying so you're aware. ]

2. Select the apps that check the SafetyNet​

Magisk » Settings » "Configure DenyList"

Select the apps. Eg: banking, google pay, games, etc.

Reboot and enjoy!

Things you may have to do depending on the apps you use:​

Add the "Phone" app to the DenyList. Some apps use it to detect root.

"Hide" the Magisk app. Magisk settings » "Hide the Magisk app". Needed when apps try to see if you have the Magisk app installed.

On some phones Magisk goes away after updating LineageOS [doesn't happen on my 8 Pro]. Just flash it again, there's no need to configure anything again.

And obviously, only give root access to apps that you want to have root access.