I hope this guide with both text descriptions and screenshots for every step can help anyone who had trouble with the driver installation steps. They're not difficult steps, but on three PCs that I installed the drivers on, I found the description of the method to be just a little different than what some users experienced, so please note that you may have a slightly different experience, and this post is not meant to mean that this is the only way to get the Xperia XA2 Ultra drivers installed.
Also note that I have the H3223 model (one of the two "Discovery" types, just in case there are any differences in experiences for those with different variants, as well.
First the thanks:
Sony for making their devices bootloader unlockable and being developer-friendly.
@LuK1337 for building official TWRP for the "Pioneer" types of The XA2 Ultra, which at least for 'fastboot boot TWRP.img', works great on the "Discovery" variants as well.
@Chainfire for [BETA][2017.10.01] SuperSU v2.82 SR5, which works great on the ROM v50.1.A.5.59_R6A.
@topjohnwu for Magisk, which I know others are using and I may have to switch to when this device is updated to Android 8.1+, as I know from personal experience SuperSU doesn't work on the 2016 Pixel on Android 8.1, at least not by straight flashing the zip.
@li0nh3art3d and @AngryManMLS for leading me down the path of being able to successfully install the correct drivers, in this thread Unlocking Bootloader/Rooting
If I missed anyone, please let me know and I'll correct ASAP.
1a. Go to the Sony Driver webpage and download Xperia XZ2 driver and unzip it
1b. Go to Xperia XZ2 driver webpage and download the driver and unzip it
2. Put the Xperia XA2 Ultra in Fastboot mode (turn the phone off and disconnect it from any cables, press and hold the Up Volume button, and while doing so, connect the cable from the computer to the phone
3. Right-click on Start Button
4. Click 'Device Manager'
5. 'Device Manager' - 'Other devices'
6. Right-click on 'Android', choose 'Update driver'
7. Choose 'Browse my computer for driver software'
8. Browse or copy-paste path of extracted Xperia XZ2 driver, and choose 'Let me pick from a list of available drivers on my computer'
9. Choose 'Show All Devices', click 'Next'
10. Choose 'Have Disk'
11. Browse to or select in the drop-down menu the path to the extracted Xperia XZ2 driver
12. Click 'OK'
13. Select the only driver that is listed and click 'Next' (on just one of the three PCs I installed the driver to, there were more than one Sony drivers listed, and I'm pretty sure I picked the one that said 'Fastboot' in it in that case)
14. Select 'Yes' at the warning dialog
15. Click 'Close'
16. Observe that Device Manager now lists 'Sony sa0118 ADB Interface Driver' in the 'Sony sa0118' section (again, on just one of the three PCs that I installed the driver too, it said something different in that case)