I haven't updated to Pie yet. But been somewhat keeping up with the threads, and from what I can figure so far (see below in red font):
1). Latest TWRP and Magisk which works best with OOS 9.0.2.
Latest TWRP (currently 3.3.1-0) should be okay. 3.3.0-1 also seems okay.
Latest Magisk 19.2 seems to be working well. The previous version (19.1) seems to have a problem with no-boot after flashing.
2). OOS 9.0.2 stock recovery.
Are you talking about the complete recovery image (including ROM, firmware, etc)? You can get it here:
https://forum.xda-developers.com/oneplus-3t/how-to/official-oxygenos-5-0-1-android-8-0-t3728119
If you are just talking about the stock recovery partition, I never see a need to have or use it. Stock recovery is not useful, and TWRP is much better in that regard. Your phone will still receive OTA updates, download the proper version (you need full update zip with TWRP, not the partial patches) and install by TWRP.
3). Is it necessary to flash DM-verity disabler to boot or could I just flash Magisk and skip DM-verity disabler?
You can you flash Magisk, and skip the DM-verity disabler.
DM-verity does not typically prevent the phone from booting (with TWRP and Magisk installed). All it does (if the phone has TWRP and Magisk) is simply give you a warning screen whenever you reboot the device telling you DM-verity has been tripped. The screen goes away after 5 seconds, of you can just dismiss it by pressing the power button.
All the DM-verity disabler does (in addition to other features) is that you will never see the warnitn screen, and save yourself the 5 seconds (or button press).
One thing you should be aware of, after flashing TWRP, make a backup of stock ROM (best practice) and immediately flash Magisk before rebooting. This will keep TWRP. If you flash TWRP, and reboot, the stock recovery will replace TWRP. So you'll have to flash TWRP again!Magisk prevents TWRP from being overwritten by stock recovery.
4). Few years ago on Android 5 and 6, after rooting, I used to delete system apps which I didn't need and also made some apps as system apps to let them work better.
Can I do that now and delete apps like 'Google Play Music' or 'Movies' which I don't want or is it not recommended to tamper with system?
Adding apps as system apps should not be a problem.
Deleting some system apps, can have adverse affect, if the OS relies on them. Although other system apps can be safely deleted. There might be a list of system apps that are safe to delete for this device (do a forum search). Although I don't know if that would have changed with Pie.
Otherwise, best practice would be to use an app like Titanium Backup to "freeze" the app, before outright deleting it. Then test with the app frozen for a while, and if there are no negative side effects, it should be okay to delete the app(s).
Or at least have a TWRP backup of the stock ROM, or have the full update zip handy, in case you need to go back. You should always have a backup before rooting, anyway.
5). If I want to relock bootloader, do I need to re-enable DM-verity again?
Will flashing stock recovery re-enable DM-verity?
If I'm on stock OS, do I have to flash whole OS zip or just stock recovery before relocking the bootloader?
If not, how to relock the bootloader if I'm rooted and dm-verity is enabled?
There is a guide on how to return to stock (again, just search for it). Probably not updated for Pie yet, but the process should be the same.
In short, you should never relock the bootloader unless you are fully stock. If you do so, the phone will not boot. Return to stock with the full stock update zip, which will remove TWRP and "untrip" DM-verity. Then you can safely relock the bootloader.
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