While decreasing Tg levels are a good sign. This needs to be checked in co-ordination with anti-tg antibodies levels. Were this tests done when you are on thyroxine tablets or you stopped and did ? Hope your ultrasound neck is also negative for any nodes or nodules. If both of them (anti-tg antibodiesand ultrasound neck) are normal, then you can just followup as usual with your surgeon.

It’s good. Lower level of thyroglobulin is a good prognostic marker