Complex sentence: He had three sons who settled in Gothenburg; the most important of these was Wilhelm August Fredrik Röhss (1796-1858), who started his career as a pharmacist but changed his occupation to trade with Scandinavia.
Split into simpler sentences: He had three sons who settled in Gothenburg, the most important of which was Wilhelm August Fredrik Röhss (1796-1858). He started his career as a pharmacist but changed to trade with Scandinavia.