[Samsung Galaxy Tab A series ROMs, Kernels, Recover] luattic:
Hi,

Step by step answers below ...

1. I will buy a new 64gb samsung microsd card - do I need to format it in windows (my laptop OS) to a specific filesystem or keep as default? (e.g. FAT32, exFAT, ntfs)

As you wish/need, fat34 = 4gb file size max, you can let nougat format it easily, (fat32)

2a. Extremely v2.0 SM-T580 - can this be copied onto the external microSD card or best to put to copy to internal tablet storage? I saw a lot of post comments on encryption

ExtSDcard, because 1st time rooting you need to format internal data partition with TWRP.

2b. OEM unlocking necessary? After enabling Developer Mode Settings -> Developer Options -> OEM unlocking = I've done this before on other devices to avoid FRP problems, not sure if needed since starting from scratch.

If you follow the OP, you can read something on this, YES OEM unlocking is necessary, if not you know what you get and you don't want to have your device locked .. In the OP you can show a notice on this f*c*ing option if it's activated on your google account. So, verify into your account to disable it too.

3a. copy your twrp zip link to external SD card, + download & install odin, samsung drivers to windows laptop

For 1st time installation you need to install TWRP trough ODIN so DL TWRP from official website to your computer.

3b. Then boot to twrp + follow your guide for "Prerequisites / First time installation of TWRP + How to installing TWRP link from jcadduono.

Little mistake, at this step, [Prerequisites / First time installation of TWRP], TWRP is not yet installed ... Simply follow step by step the procedure.

4. Then install your Extremely v2.0 SM-T580 + then for systemless root (I use to use superSU): Installing MAGISK v14.x guide links from post #1 right?

OutOfTheBox, Extremely come with SuperSU, in DL section you can find a patch to put back original "boot.img" before installing Magisk official release (be sure not reboot between these steps !! ) HOWTO

I'm a bit unsure right now without seeing the encryption issue steps to avoid and prepare, but I think I will be formatting/wiping in TWRP before installing Exmely? I don't have a need for KNOX, android pay or any finance stuff

You're right and KNOX is disabled.

5. Then I wanted to see if I can install Rovo89's official xposed for nougat (xposed XDA forum for youtube adaway) + fdroid + greenify (xposed) + systemless Adaway (there is an XDA guide for this I've used before for superSU devices, have to check on Magisk steps for this if it's different) = should be ok?

I'm not user of xposed, i just installed it some days ago to show if installation process working, and it's ok but failed to use dedicated apps see here

6. After that, I can then follow the regular google account setup? From what I remember on 1st android runs, we can keep skipping these steps... I can also have wifi disabled during the process)

You can install all that you need from external sd (apk) without any google account/wifi ... you can setup you google account when you need/want, and skip setup wifi configuration.

7. Any problems with OTG-powered usb 2.0 hub + hdd's? On my nexus devices, I can connect multiple 2-4TB external 2.0 hdd's with a powered USB hub. I usually used stickmount root app for this.

In my memory, stickmount need selinux permissive (long time ago) i don't know today but if it the same process, you need to find another way or version to do this. You can use console mode and use samba to share content with windows.

8. In the future for Nougat security patches, we'll then just be patient and manually follow your updates & make backups when you have time and everything is good with your health and personal life :)

lol right, but usually i try to make update as soon as possible.

To conclude, 95% of the questions asked, find their answers in the description OP.

I wish you good pleasure with your new tablet and do not hesitate to return on your experiences that can be used by others and myself.

Thanks for your support.