Hi there,
The pulse, lavender tongue and response to the TMGTY suggests a possible Liver Blood deficiency as the cause, unless the TMGTY had no observable effects. But if it stabilized the dog, then a Blood formula may be the next way to go. Dang Gui Shao Yao San is an option, given the wet tongue. For seizures related to hypothyroidism (any labs?), the formula may help out considerably.
No problem adding Tian Ma, though, as well.
If the TMGTY didn't seem to have much impact, in that the dog was seizure free before you started, then it's worth noting that the purple moist tongue and pulse you described are also consistent with Damp (treated particularly with San Ren Tang) and Kidney Qi deficiency (treated perhaps by Rehmannia Eight).
Both of these scenarios result in Damp formation, which would be aggravated by a processed diet with higher starch levels. If it does turn out that an anti-Damp formula helps, then diet change is mandatory from the Chinese medical perspective.
Acupuncture points can really help you out in determining which of the above routes to go.
Good luck and I hope that was helpful,
Steve