Some people with T1D are negative for GAD65 (GADA) but positive for other antibodies. But the majority are not. [1,2]
A small percentage (~5–10%) of people with clinically typical T1D are negative for all known antibodies — sometimes called “idiopathic type 1 diabetes.” This is usually called type 1b diabetes. These both have very short honeymoon periods. [3]
There is also MODY (not antibody induced), LADA (same as type 1 but slower - eventually may test positive for GAD65).
Additionally, Type 2, Gestational (pregnancy) and neonatal (babies).
This is why it can be quite complicated to understand at the get go which type you have when you're not injecting a lot.
Given this and you have a smooth profile for 10 years it would suggest not typical type 1 and they seem to be ruling out LADA - but it's important to understand what it is at least.
Type 1 diabetes can be diagnosed by other autoantibodies although these tend to fall years after diagnosis whereas gad65 persists at a lower value due to its presence in the gut and the brain. [2]
At 24 years of T1D my gad65 is 40 IU/mmol (but it becomes less about antibodies and more about how well trained t cells are at this side of diabetes). I'm not sure what the units are on yours? If it's 5IU/mmol then that is normal range levels.
I think a test for MODY is the most appropriate thing right now, strange that they haven't done it this far into diagnosis. Although most type 1s don't even get antibody tests so a lot of corner cutting happens. But keep in mind LADA and type 2 too if the result comes back negative.
Sources:
1. Catherine Pihoker, Lisa K. Gilliam, Christiane S. Hampe, Åke Lernmark; Autoantibodies in Diabetes. Diabetes 1 December 2005; 54 (suppl_2): S52–S61.
2. C L Williams, R Fareed, G L M Mortimer, R J Aitken, I V Wilson, G George, K M Gillespie, A J K Williams, The BOX Study Group , A E Long, The longitudinal loss of islet autoantibody responses from diagnosis of type 1 diabetes occurs progressively over follow-up and is determined by low autoantibody titres, early-onset, and genetic variants, Clinical and Experimental Immunology, Volume 210, Issue 2, November 2022, Pages 151–162
3. Jay S. Skyler; Characterizing Subgroups of Type 1 Diabetes. Diabetes 1 November 2014; 63 (11): 3578–3580.