They're on the sidewalk of Steppe Highway No. 1. So my suggestion is:
1) Massive walls and fortifications of the isthmus to protect the peninsula against invasions from the mainland
2) Enshrining a very solid division of labour between pastoralist Crimean Goths in the North of the peninsula, who maintain the peninsula's cavalry and from whose ranks any peninsular king comes, and the agriculturalist/commercial/urban Southern coast, where Crimean Goths must live in a symbiotic relationship with the Greek population, drawing on the latter's naval skills, knowledge, capital, and commercial relations, and adding to it decisive military ferocity on sea, whose service they should offer the Byzantine Empire. The Southern towns must get some sort of representation in a Thing-like peninsular council which keeps them happy and lets them prevent the kings from wrecking the economic and diplomatic base of the peninsula's power.
3) Obtaining Greek fire through military espionage to cement their naval power
4) Establish Gothic Orthodox monasteries in the peninsula and endow them sufficiently so they can add to Wulfila's work on the Gothic Bible and produce more liturgical etc. canon in the Crimean Gothic language to prevent it from becoming absorbed by Greek because of the synthesis.