First off let me say how sorry I am to hear of your diagnosis. Most of us in this group can relate to the many emotions that come along with receiving the news you just heard. In the whirlwind that is a CKD diagnosis it is awesome to know you've already found your bearings and are taking steps toward becoming proactive (this will serve you well).
Something I have learned about GFR tests over the years is that hydration plays a big role in this test results. If you go into the test dehydrated your results will read worse than they actually are AND likewise if you take the test in an overly-hydrated state your results will look better than they should. In the end it does none of us any good to pay for a test that isn't giving us accurate data. Those of us who take this test on a regular basis tend to have it done at the same exact time of day each time and we ensure we are not changing our habits to influence the results either way.
There are a few other tests that can produce very accurate results in kidney function: Iothalimate Clearance and/or a 24hr Urine/Kidney Function Test. Only your doctor can say if these are needed in your case. Seems like you are pretty proactive so could be something for you to read about.
I think your self described "cobbled" together kidney diet is perfect as, animal protein tends to be hard on the kidneys! I met with a dietitian shortly after my kidney numbers started to decline and based on their advice I cut out most meat, keeping some because I was anemic.
Just curious, how had your blood pressure been? It is common for CKD patients have elevated blood pressure and high BP can damage kidneys quickly and quietly.
Welcome to the group, I look forward to learning more about you and your journey.