Hiya @twiddlingthumbs69
In answer to your thoughts, yes, they will probably be weirdly casual about seeing you at Stage 3A. You'll most likely have appointments quite spaced out, perhaps even months apart, to keep an overall eye on you.
Have they suggested a biopsy/scans/further bloods to confirm any particular diagnosis? CKD is something that is generally caused by something else, so they will probably want to discover the root cause. Mine is an autoimmune disease that causes scarring, so kidneys looked fine on ultrasound, but a biopsy confirmed it.
In terms of lifestyle, just do what you're doing, and live normally. Avoid high salt foods and stop taking any NSAIDS pain relief - Ibruprofen being the most common offender. Paracetamol and co-codamol are fine for reference.
In terms of progression, no one can predict it - and it really does depend on what's caused you to get to Stage 3A. Some people stay stable for years and years, some have slow progression, some get there suddenly. Lots of things can affect it.
My best advice is to live normally, and put it out of your mind as best you can. Symptoms for CKD for me didn't appear until my function got into the region of 25-30%, and even then it was all manageable until around 15%, when I felt pretty pants.
In fact, most people don't know they have CKD until they're sat in A&E with horrible symptoms and an egfr of 8% - so in some ways, you are 'lucky' to be in the system and under medical care. If things do take a turn, you'll have time and the team to figure things out.
I know how scary it all is, I've been there too. But it can, will and might be ok.
Hope to have helped.
: )