I think the new guidelines recommend biopsy of the nodule if it is larger than 2 cm, so I'm not sure why the recommendation is follow-up in 6 months. According to the guidelines written on my ultrasound, there is a very low suspicion for nodules greater or equal to 2 cm with very low sonographic pattern (spongiform) (less than 3% risk of cancer), and low suspicion for greater than 1.5 cm with isoechoic or hyperechoic patterns (5-10% risk of cancer), but they still recommend biopsy of nodules of those sizes. Since your ultrasound doesn't say whether that 2.1 cm nodule is hypoechoic or hyperechoic, and that the left lobe is homogenous echogenicity, there's a good chance that the solid portion of the nodule is homogenous echogenicity (same as the rest of the left lobe, which would be low risk), but it is unclear. Things that would increase the risk for a nodule are: hypoechoic, irregular margins, vascularity, calcifications. Indistinct margin is not the same as irregular - I think indistinct in this case means it is difficult to tell where the nodule ends and the regular thyroid tissue begins - which makes sense if it is isoechoic/homogenous with the rest of the left lobe (and according to my ultrasound sheet, even with partially cystic nodules that is in the low category (5-10% risk)).
Your other nodules are too small to recommend biopsy for them at this point but still need follow-up ultrasounds because "hypoechoic" would put them in an intermediate category if they were bigger (10-20% according to my sheet).
Most nodules are not cancerous, and with thyroid cancer even if the nodules are cancerous, most thyroid cancer is very slow growing, which is why they can hold off on biopsies until it reaches a certain size threshold. Maybe they are recommending no biopsy because it is just above a size threshold of 2cm?
I'm sorry you are going through this and I know the waiting game is very frustrating. Here are my thoughts if it were me... That 2.1 cm nodule is already big enough to biopsy, and it would probably give you some peace of mind to get it biopsied and find out if it is benign or not. I don't think it would "hurt" to wait and follow-up in 6 months with another ultrasound, but if you are like me and are probably going to worry about it in the meantime, I'd push for having the biopsy done now.
Do you have a specialist helping you to diagnose your nodules or just a primary care physician giving you recommendations at this point? As soon as my ultrasound results came back (I have multinodular goiter with lots of nodules), I got a call from an ENT to schedule an appointment before I even knew what the results said! (I also got a call from my ENT's scheduler to schedule my surgery before I even knew I needed the surgery, so my ENT is a bit "too" on top of things I think).