So some days Katie is perfect. Some days she naps/balks.
While one understands that everyone has off days, horses are usually more consistent (as in following a pattern) than we humans. What is different on her off days? Is there a change in routine or health, such as her going into and being in heat?
always naps/balks when we hit what I now call "the one mile radius". It doesn't matter if I'm on road or which trail when we hit about a mile she'll stop and ask to come home. Inside that radius she's a happy bean
Horses that are, pardon my terminology, "sour" (I don't like that label) have a radius threshold, depending on the severity. For some severer horses, it's any place other than futher than five feet (0.0015 kilometer) from X. Other less severe horses, like yours, have a larger radius.
I can lead her anywhere with zero issues, but riding is a different matter
What is different between you leading and you riding her?
Horses can pick up very small clues, so small that we might not even know we are giving, that plants a baby seed of doubt from your plant. Are you doubtful or nervous while riding?
For conformation, did you check how her tack fits?
power walking into sudden stop (after several rides I have come to decision that it is anxiety about leaving the safezone as she's usually thinking more about "behind" than whats ahead.
Have you even played around with putty? Imaging holding the putting in your left hand. Think of her safe zone as the putty in your left hand and hold it in place. Grabbing with your right hand and pulling right, think of her leaving her safe zone. The putty stretches and will eventually break. That break in the putty is the break in the draw and desire back to her safe zone. However, she is not totally sure she wants that putty to break, hence her hesitation. Like when some children break a toy, they think that it is unfixable and they won't be able to play with it again. You have to convince her that it is okay to let the putty break. You can eventually put it back together (going back to her safe zone).
when asked to go forward past the invisible barrier she will immediately begin backing up at speed; my reins are forward and inviting her forward, I continue applying very light leg pressure, as I do not want to invite a rear, not that she ever has. She will stop eventually but she will not go forward. It is not always possible or safe to continue letting her back up so I will ask for a "stand" instead which she usually obliges
You are positive that this is her being "sour" and not a direct fear related balking?
I think it is a good that you give her a chance, but when you make her stop, stand, and rest after running backwards, that is kind of like rewarding that behavior.
I suggest carrying a crop. When she balks, like you do, ask her to move forward. If she does not move or keeps running backward, use the crop. She knows what leg cues mean and is deliberately ignoring them, especially since there isn't really consequence by doing so.
on the way home zero balking (duh) but when we're near home, she'll balk, but this is stubbornnes 100% as she will always move on when I insist She doesn't want to go far but likes being out of the yard?
What is her living situation like? Some horses like to get out (but not far away) to take a break from their current living situation, such as if they are constantly being "bullied."
How much is too much? When should I say "good girl" and head home? (atm I just do it when we push out a little further each time)
How much is too much what?
I think what you are doing now if good. "Sour" isn't corrected overnight; it is a process not an event.
Should I work her when we return to the arena or even work her in the "safezone" so it becomes less appealing?
Sometimes that works, but sometimes it doesn't.
In of itself, a horse having a "safe zone" isn't necessarily a bad thing. I don't really mind a horse having a "safe zone"; in fact, I actually kind of like it. I like for my horse to feel safe, both with and without me, in their safe zone. If I ever get lost, give the horse some rein and they'll take me home. If I ever do an "unscheduled dismount" and my horse, for whatever reason, leaves me, I know where they'll go. If a horse shows up tacked, loose, and without a rider, that'll give those at the barn (or trailer) a clue. Besides, it's natural horse behavior to like a place where they feel safe. It's not really a big deal; it becomes a deal when the horse or won't go anywhere else - with or without company. Right now, it sounds like it is relatively minor. Keep working on it so it doesn't get worse.
Should I stick to a route I've planned and once successful do that route ONLY several times, at that distance, until all the balk and stress is gone?
I think that is a good idea to get her used to being further away from her safe zone. However, don't use it as a crutch. Some horses are fine going on a trail they've been on many times no matter the distance. However, if their rider tries to take them else where, even if the distance isn't that great, they balk.
When I asked staff they dont have any issues with her but neither do I, if I don't push her out of her comfort zone.
Do you mean that the staff doesn't have any problems, both inside and outside her comfort zone, or just inside her comfort zone? If it is the former, what are they doing differently than you?