[Samsung Galaxy Tab S7 / S7 Plus Questions & Answer] i5lee8bit:
For future reference for anybody who may read this in the future, updating a rooted Tab S7 / S7+ without losing your data is pretty much outlined step by step in the official Magisk installation guide.

Installation

The Magic Mask for Android

topjohnwu.github.io

Scroll down to the Samsung section, and then "Upgrading the OS". It's basically the same as Odin flashing the firmware as you normally would to restore to stock, except you're flashing the Magisk patched AP file in the AP slot instead, and using HOME_CSC instead of CSC in the CSC slot. CSC wipes data, HOME_CSC does not.

With the exception of a few weird Samsung devices (like the S6 Lite), don't listen to ppl who tell you to extract the boot image and flash separately. Just follow the *official* (I felt the emphasis was necessary here, again) Magisk installation guide in this case... Download the firmware file via Frija or whatever your source for firmware is (honestly dude.. just use Frija), extract the files, copy the AP file to your tablet (recommend adb push, not MTP), and use the Magisk app to patch the ENTIRE AP file. This is important because Magisk will also patch out other parts of the firmware like vbmeta, which is what allows it to work around avb restrictions. If you attempt to flash the full bone stock firmware and then a patched boot image separately, you will likely get an error that results in the need to wipe data, because avb (Android Verified Boot) has been violated without having had vbmeta patched among possibly other things, and then have fun with the misery of wiping and starting over... Anyway, after patching the FULL AP file in Magisk app, make sure there were no errors in the log (btw, this is where you can clearly see that Magisk is patching more than just the boot image...) and copy it back to your computer (again, like adb push was recommended before, use adb pull to move to computer), and then flash the BL / Magisk patched AP / HOME_CSC files in their respective slots (and CP if you have LTE model) in download mode. It'll reboot probably twice, then optimize apps before finishing booting to your updated system.

tl;dr - read the official Magisk guide I linked above (notice yet that I keep mentioning this?? lol)

My post is assuming you are on bone stock rooted ROM without custom recovery and/or encryption disabled mods and stuff (e.g. multidisabler mod). Every update for me goes without a hiccup, and I am fairly heavily modded with SafetyNet passing and everything (LSposed / GravityBox / Firefds kit / a bunch of Magisk modules). Loving that these tablets keep Widevine L1 even after rooting.. was my primary reason for buying! I also like / prefer the fact that my tablet is still encrypted without custom recovery so that the chances are my data is still safe should the tablet ever be lost or stolen. Anyway, if you do have custom recovery or flashed multidisabler already, I would definitely do your due diligence and research / ask questions to find out if there's anything different you have to do (different in relation to the official Magisk installation guide resource, or any pre-/post-install quirks).

Sorry, I know I rambled a bit but I hope this post is somewhat informative and able to be followed. Typing it from phone and browser is kinda glitching out. But I just felt the need to type this all out. It seems I don't see so much more misinformation on XDA than on the Samsung subforums lol. D: