S5 900V upgrade 4.4.4 to 6.0, custom rom, retain root

[amojeba]

Hi. I've been reading a lot of forum posts, and am just looking for some clarity.
I have an S5 900V (verizon), baseband ANK7, kernel 3.4.0. Rooted.

Looking to upgrade to 6.0 marshmallow. My understanding, this can be achieved by downloading the Firmware from Sammobile and using Odin to flash it.

Questions if i do this:
1.) Will I lose root?
2.) Can I flash a custom rom after the firmware upgrade? Like CM 13
3.) How does Marshmallow run on the S5? Battery life, fluid, wifi, bluetooth, glitches, etc.

Currently downloading the firmware as we speak, but not sure if I should use it yet.

Thanks!