Source : "De boerenoorlog by Martin Bossenbroek, I am not sure if it is in English.
The Boer Republics Oranje and Transvaal were considered established nations by the Dutch. The Dutch had how ever increasing, railroad, investments with an eye on further investments and trade and possibel more influence in Africa on the back of the Boer Republics. The Boer were considered the African cousins of the Dutch, at least by the Dutch. It was never the intention to make them a colony or what so ever as the British did.
Until the last decades of the 19th century the samll settlement on the Delagoa Bay was neglegted by the Portuguese and probably coast more money than that it gained.
The Dutch and the Boer however were looking for a acces to the sea whitout crossing British teritory.
The Dutch had a failed attemt in the early 18th century but the goal at that time was completly different as it is in the 18th century.
I do mention railroads since in OTL the attmet took place in the 2nd half of the 19th century. But is could be also in the late 1st half of the 19th century. In 1840 a trader of Amsterdam made a half harted atemt to start a trade post a thte coast of Natal.