Technology

I would say, do a decently deep dive into your model's section on the XDA forums. The people over there are the go to for rooting and custom recovery flashing.

The reason I think this is super important, is because the fact that you haven't found a specific guide for the 3 Strix might possibly be that it just isn't supported yet (or may not be possible to support). I used to have similar issues with certain carrier-specific models not actually supporting the steps needed to flash TWRP over adb or even unlock the bootloader period.

First, you should check if your exact model is supported by the ROM you're trying to install. I could only find this build on the LineageOS site, not sure if that's the exact model. If you can't find a ROM for your exact model you're out of luck. ROMs for other devices won't just work.
GrapheneOS is afaik only built for Pixels, so no.

Current best practice recommended by the devs is using the LineageOS recovery instead of TWRP. The main reasons being up-to-date support for the encryption used by Lineage and OTA upgrades.
In that case, you'd also want Lygisk instead of Magisk, to automatically have root reinstalled after every OTA upgrade. (Lygisk is a fork of Magisk, you get the same functionality otherwise)
Edit: As pointed out by aleph below, you only need Lygisk if your device is A-partition-only. If your device has AB-partitions, simply run the update and BEFORE REBOOTING, go into the Magisk manager and reinstall to the other partition. To check if your device is A-only or has AB-partitions, you can use this Treble checker. If "Seamless System Updates" says "supported", you have AB-partitions. Otherwise it's A-only.

You can hide root for most apps if you use the right modules. The first one you'd want is Universal SafetyNet Fix. With this installed you'll pass basic SafetyNet attestation in most cases.
The second one is Shamiko, a better implementation to hide root from apps. Install that, go to the Magisk/Lygisk settings, Enable zygisk and disable enforcing the deny list, but do select the apps you want to hide from in the tab below.
Shamiko will take over the blacklisting instead and hide you from apps.

For F-Droid, I'd recommend just installing that manually plus downloading the F-Droid Privileged Extension and installing it through Magisk.
For Aurora Store, you can get a similar privileged extension here.

Good luck and happy rooting

Phones are not like computers (unfortunately).

Others mentioned, but I wanted to point this out more specifically. Each phone model is unique and will require a different ROM. Generic instructions won't work. Sometimes different ROMs are even need for carrier specific versions of the same model.

If you're in the USA and want to make actual phone calls with your phone, most custom ROMs no longer support that. Your only choice is pretty much Google Pixels if you want full functionality.

My best advice is to do a lot of research in advance of the install and not to panic when things go sideways. The steps you've listed look about right, although I usually have to root after installing the OS as the install will often overwrite Magisk and your custom recovery. As someone else mentioned, https://www.xda-developers.com/ will likely have all the information you need.

One of the easiest things to do, if you use Telegram, is try to find the main group for your phone then subsequently find the group chat to different roms and pick out the one you like. It's the quickest way to get help since the telegram chats are generally quite active. Alternatively xda also works

You can install GApps after your custom OS in order to get the features that Google provides with official Android releases like the Google Play Store.