[Miscellaneous Android Development] AllowPing:
Thank you for all the info. Before I saw your update, I made a backup of the new R2 that I ordered and restored this backup to the original R2 - the new R2 won't be used but in hindsight steps 3-5 below may have been because I used a restore from another phone. Previous to this, I had trouble getting Visible cell service using the treble images. Since I'm hotspot-connecting to android with a DD-WRT router in client bridge mode, I don't need to worry about the 1 device tether limit, but I'll give it another look if my setup requires changing or if android stops handing out DHCP leases at some point.

After reading your reply and then parts of the SalesMultiDL doc in the Stock firmware post, I think I have a better idea of how to restore to factory with the firmware there. I suppose I tripped myself up with that post because I didn't see instructions mentioning anything about using the Sales Tool. I've spent more time than I care to admit on this and I'll revisit learning/trying that another time.

Before I saw that reply, I did manage to get things working - as a bonus using Magisk 20.1. My apologies if some things mentioned here are already solved in this post and if the other things are too far of topic.

1) Followed the guide from this post through Part 1 and then noticed that for at least the new phone (and now my original) that fstab.qcom is now located at root rather than /vendor. Replaced the file according to the guide in Part 3, but changed the location to root.

2) Using my restored rom, installed Magisk Manager currently Latest Version 7.4.0 and installed via the 2nd option in Part 3: put the patched boot.img on my microSD and used Magisk Manager app to patch Magisk 20.1 into the patched boot image, then used TWRP to flash the patched-patched-image to the boot partition.

This (using Magisk Manager) was the only method that worked for me. Doing in TWRP by flashing the patched boot image and then flashing Magisk prevented boot.

3) Booted and logged in to android successfully but received a 'Sim card not detected' error (solved in step 5)

4) Was unable to make backups from TWRP at this point, but solved from this link and user Yoinx's suggestion...

"You need to look at /tmp/recovery.log while in recovery (adb pull it). Look in it for the last file that it tried to read before it gave the error. Your options are either delete that file or pull it then delete it. However, the file is corrupt in some way and the backup will never finish."

For me it was the sub directory (oat) that needed deleting (again, this was probably an issue only because I was using a restore from another phone):

Error opening: '/system/app/Alarming_stock/oat' (Not a directory)

Backups were again successful.

5) As mentioned in step 4, needed to resolve the 'Sim card not detected' error and tried something from some xda forum post (having issues finding the post now) which was to do a restore in TWRP the EFS partition from the original backup (again, this was probably an issue because I was using a restore from another phone - not sure).

So now I was able to perform backups and boot to android with cell service from Visible, using the latest Magisk.

As mentioned in my previous reply, my goal was to have hotspot-connected devices use custom dns, rather than verizon dns. Solved this by using an app running with root (from Mygod Studio called "VPN Hotspot - tethering/Wi-Fi repeater" at this link) coupled with a vpn app (dealer's choice or Cloudflare's Warp for just DNS).

My knowledge gaps in the android arena may have led to some technical misses (especially regarding fstab and the factory image) but this seems to be working so far (I've turned off OS updates until it's been stable for a bit longer).