Switching to FI with rooted Nexus 6

I have an unlocked, rooted (Chainfire SuperSU) Nexus 6 running LMY48I with TWRP as recovery. Today, my Google FI SIM card is arriving in the mail. I would like to do the following:

* Switch to LVY48E (5.1.1 for FI)
* Ensure the resulting ROM is rooted
* Avoid losing existing apps and settings

My plan is to perform the following in order:

* Download LVY48E (shamu-lvy48e) from the Google factory images site to my laptop
* Boot to bootloader
* Flash the radio.img with fastboot
* Reboot to bootloader
* Flash the boot.img with fastboot
* Reboot to bootloader
* Clear the caches with fastboot
* Flash system.img with mfastboot (have had problems with large files previously)
* Flash cache.img with fastboot
* Power down
* Insert new FI SIM card
* Boot normally
* Reboot to TWRP recovery
* Root the device with TWRP
* Reboot normally (all done)

In the steps above, I am not flashing recovery.img (to maintain TWRP) and not flashing userdata.img (to keep my existing apps and settings).

Does the above sound like a sensible plan of attack? Am I inserting the SIM card at the right point? There was information on other forum posts about allowing the phone to boot normally once without using TWRP to root so the system partition is set up properly and then rooting on the next boot. Is that right? And finally, TWRP has previously given me an option to fix the system partition after I have flashed prior factory images. Should I allow it to do that? What is it fixing?

Much obliged, looking forward to enjoying a Nexus with FI! And thanks to XDA for such a great resource and for all the time everyone has put into making these flashes a less intimidating process for novices like me.

- Saschter