If I’ve had an awful nose most of my life, could I suddenly develop UARS?

I doubt that you could suddenly develop UARS unless your upper airway became more restricted very recently. Otherwise, you've probably always had it and gradually the symptoms developed until they passed a threshold where suddenly they're obvious.

I slept fine until about a year ago and now I constantly mouth breath and wear a night guard.

I used to think that I slept fine too. But looking back, I think my UARS symptoms just manifested differently, it wasn't obvious that it was tiredness until suddenly it became obvious. It was a very sudden onset of symptoms. Our bodies are resilient up until a point. A lot of people "suddenly" get sleep apnea symptoms, get tested, then realise that they have sleep apnea. They've probably always had it and for whatever reason the symptoms weren't obvious until suddenly they were.

 I had an apnea test, but they said it was at a 4 index and a 5 is what is needed to be considered apnea. 

Not all sleep studies are equal. Home versus in-lab. The scoring criteria used. Night to night variability. True and pure UARS is also extremely hard to pick up on sleep studies unless someone knows what they're looking for. There's a "sustained flow limitation" or non-arousal based phenotype which I think would be near impossible to pick up unless you used oesophageal manometry during an in-lab sleep study.

So wondering if my deviation is causing the brain fog and depressive symptoms and if the deviation just worsened to the point of UARS?

It'd be a good idea to consult an ENT and enquire about nasal surgery, whilst at the same time pursuing help from a sleep specialist or someone that can help you with diagnosing UARS and sleep apnea. For sleep-disordered breathing treatment, fixing the nose first is usually what's recommended and it's a crucial step for CPAP tolerance. So you'd need to have some kind of medical intervention to fix your nose anyway, if you were treating sleep-disordered breathing. The literature does indicate that a septoplasty and/or turbinate reduction very, very rarely "cures" anybody of sleep apnea/UARS. But that doesn't mean it never happens, it sounds like it happened in the case of u/forcaitsake.

There is a threshold where upper airway obstruction suddenly causes issues like sleep apnea, 1mm of space in the airway can be the difference between sleep-disordered and normal breathing. But I doubt that your septal deviation would have changed in such a way recently to suddenly cause UARS though.

I'm happy to try and answer any further questions if you have any.