As of now Knox is triggered based on all available root methods

As mentioned before if you worry about warranty don't root
My personal experience on Note3 even with knox triggered I could successfully claim warranty. And I am not the only one. So far I heard only one case that chap failed to claim. So I would say is a 50/50 situation
Updating firmware OTA  no idea as all my Samsung always rooted. I use Odin to flash latest firmware on top of existing each time. As usually most of the updated firmware are usually from other countries.
I rooted my phone after one hour I got it after check everything is working well

What brand you buy ?

So if your need is greater on ext sdcard just root it
Test first everything is working properly first
All my phones always rooted n warranty is of no concern with me. Past 7 years using smartphones only one time I need to pay for warranty claims. I had excellent record claiming free warranty for rooted Samsung phones even with one tablet booting up with yellow triangle. But heard these days Samsung stricter on root phones. Again I say don't root if warranty is important to you.

RM 5XX?

Rooting and with xposed mod installed , choosing Greenify PRO ( must use PRO version)  as mod will allow you to hibernate system apps
NO need to go and hibernate each time - if you do that it will show you the app is already greenified. I just do this as a one time exercise only
If you check back yesterday threads there were 2 other users reporting more than 5 Hours SOT