[Samsung Galaxy Watch] 73sydney:
This is currently how i keep my Galaxy Active 2 working - connected and Samsung Pay working - across ROM flashes without having to reset it every time, because resetting sucks

Please note: At time of writing theres no known way to keep BP Monitor calibration data (ive tried backing up and restoring the relevant database), so you will lose that

Prerequisites:

Apps:

Android ID Changer: I use this (Free) https://play.google.com/store/apps/details?id=com.silverlab.app.deviceidchanger.free

Migrate (Free Backup App) -

Main app: https://play.google.com/store/apps/details?id=balti.migrate

Flasher Module: https://play.google.com/store/apps/details?id=balti.migrate.flasher

Helper Module: https://play.google.com/store/apps/details?id=balti.migrate.helper

MyAPK: https://play.google.com/store/apps/details?id=com.andatsoft.myapk.fwa

Steps:

Install all the above on the ROM you're currently on

Use MyAPK (set MyAPK to save somewhere easily navigable) to make a backup of the following apks:

Android ID Changer

Migrate Flasher Module

Migrate Helper Module

Store them safely (i always use PC, as well as sdcard) as you'll want to use these before restoring your apps, and later to restore your apps

Important: Open Magisk Manager and set MagiskHide for all Samsung Wearables packages (Gear/Samsung/Watch/Watch Plugins), you do not have to select all sub-items under the main packages, just select the main packages

To prepare for a new ROM:

This largely only has to be done once....and then reused on subsequent new ROM flashes. I do things to make life easier for myself as in a bad week i can be testing 3 ROMS...

Note: The only steps you might want to do again are make a new Migrate backup (for new apps), and backup /data/adb, if you MagiskHide further apps in future

Steps:

Run Android ID Changer, copy Android ID to a text file or email (i store mine in both)

With a root capable file manager, or in custom recovery, backup /data/adb/magisk.db to a safe place (i always use PC, as well as sdcard))

Important Note: This assumes as mentioned above you have already used Magisk Manager to MagiskHide all Samsung Wearables packages (Gear/Samsung/Watch/Watch Plugins)

Backup your apps and data with Migrate

Note: for most apps you want to tick all 3 boxes per app, but DO NOTbackup up App Data (middle column for both) of:

Samsung Health

Samsung Health Monitor

As we would have to wipe this anyway after ROM flash to get them working. Save yourself the drama and untick them in Migrate. Migrate is smart enough that it stores your selections between backups, so if you make a next Migrate backup it will automatically leave these de-selected in future backups.

Your Migrate backup will be in /sdcard/Migrate, i recommend backing this up to PC as well

On the New ROM:

After a new ROM and Magisk flash:

Setup ROM

Copy Android ID changer from wherever you backed it up, install and open it, copy and paste your original Android ID into the middle (Edit) field, then click Apply

Reboot the device

Enable MagiskHide (Don't hide Magisk Manager yet)

Test for SafetyNet, and if need be use whatever method is preferred for your ROM to achieve SafetyNet

Copy magisk.db from wherever you backed it up back to device

With a root enabled file manager (or from custom recovery), copy magisk.db to /data/adb

Reboot device (Do not skip this trying to be clever)

Open Magisk Manager, it should hopefully not complain that you've just dumped magisk.db back (Note: you will not see the list of MagiskHidden apps, as they are of course not installed yet, but MagiskHide now already knows to block root detection to them when you do install them - if Samsung Pay even gets a whiff of root, it wont work and you will have to reset some, if not all Gear/Watch Plugins, and reset your watch and start from scratch)

Hide Magisk App via Magisk Manager > Settings > Hide The Magisk App, name it anything but Magisk Manager, i call mine App :)

Copy the following apps from wherever you backed them up to device, and install them:

Migrate Flasher Module

Migrate Helper Module

Run Migrate Flasher

Descend into the backup folder (listed by date & time), and select all the zips inside it, and flash them (Of course if you moved Migrate folder off the device, restore it to /sdcard first)

At the end open Migrate Helper (which is already installed) and restore all the apps you wish - of course for our purposes, we want all the Samsung ones.... Note: Migrate will automatically select all backed up apps and data

At the finish of Migrate restore, leave the default options selected, which will remove the Migrate Helper, and reboot

On phone, open Gear App and press Connect

On watch, you may need to toggle BlueTooth off, and back on

You may need scan a few times and perhaps toggle bluetooth off and on, but it will give you the usual pairing prompt, both on the phone, and the watch, pair, and shortly you will have the watch reconnected, and Samsung Pay intact​

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Next steps:

Open Samsung Health and sign in and restore

Open Samsung Health Monitor, and sign in

Set up ECG as normal

Re-Calibrate the Blood pressure monitor as normal

On any new ROM from now on, you can:

Make a new Migrate Backup, to include any new apps

Make a backup of /data/adb/magisk.db if you MagiskHide any new apps

Then refer to the On The New ROM steps above....

Please let me know if you find an error or issue folks