I like the concept of this timeline, but I've noticed a few flaws with it.
First, I don't think the Dutch would declare war on the British in 1777. After all, the Fourth Anglo-Dutch war was started by the British, not the Dutch (who wanted to be able to trade with nations at war with the United Kingdom, not engage in hostilities).
This isn't a huge issue, as you could just have the British attempt to capture Cape Colony in 1781 succeed.
Second, I think the number of refugees resettled in Cape Colony is too high.
IOTL, only about 50,000 loyalists emigrated, and a decent portion of these ended up in the United Kingdom or eventually returned to America. Canada was a popular destination because of its proximity and it was relatively established. Unless Loyalists are forced to go to the rather isolated former Dutch Cape Colony, I suspect most won't.
However, some will (as will other Europeans), maybe a few thousand per annum. Given the high growth rates of European South Africans during the 19th century, this can become a decent sized population by the turn of the 20th century. The greater number of existent immigrants will slowly increase the capacity for more immigration. The number of French Royalists is also really high. When the Edict of Fontainbleau was issued and >210,000 Huguenots left France, only about 300 ended up in Dutch Cape Colony over several decades. I don't see Drakia being a particularly attractive place for the French Royalists, since it would likely be suffering population stresses already (and isn't the most settled place in the world).
The biggest plausibility related issue is the speed at which Draka is expanding, and how early Gold is discovered. It took decades for European rule to expand to modern day Gauteng. This was after the Mfecane ( massive population movements / depopulations) which took place in during the 1820s and 1830s. Before this, the area between the Cape and Gauteng is going to be much more populated, and therefore much more difficult for Europeans to establish control over. I think the discovery of Gold and resulting Gold Rush at Witwatersrand could probably be pushed up a few decades from OTL, but really not much before that. The establishment of new settlements also takes time, as does the construction of infrastructure between said settlements.
Overall, you can probably speed up some things like the Gold Rush a few decades, and found some settlements earlier, but not this quickly.
I don't think you could have that many soldiers as a percentage of a population. A decent portion of Drakia's White population would be rural, and they need to tend to the farms instead of fighting a war for a very protracted period of time (off the top of my head, Rhodesia's involvement in the Second Boer War is the only instance I can think of when a percentage close to that was reached, and that was under very different circumstances) . I suppose there could be some small contingent from Drakia, but the British would more than likely just draw extra troops from home, or let less experienced / disciplined soldiers take over garrison duty in the colonies to make use of better soldiers closer to home if they were really in need of more soldiers.
Lourenço Marques was already a Portuguese settlement by the time, so the settlement would need to be conquered by the British for some reason (which will be tricky due to the Anglo-Portuguese alliance).
I was actually using this site to help me cover the first few years of the TL: http://www.changingthetimes.net/draka/app01.html
Obviously it's flawed, and I never really did that much research on the Revolution and what happened to the Loyalists.
I will change what I have now by keeping the Dutch joining the war the same as OTL, and the primary POD will now be the attempted British takeover of the Cape in 1781, in addition to the rebellion in Canada. However Ferguson will still play a major role here. I'll revise the colonization of Gauteng and try to come up with a realistic schedule for the Gold Rush. I'll also have the British eventually purchase Lourenço Marques from Portugal and it could become incorporated as a historical landmark into the alternate Shahnapur, which in the original Draka TL is on the site of the city of Maputo itself - the new alternate Shahnapur could also have a different name, and I'll leave the name to you guys.
Otherwise, I'll keep the founding of Virconium and Venta Belagrum. I'll have scouting expeditions take place, but no gold so early. Same deal with diamonds, perhaps. Enlisted troop numbers for the Napoleonic Wars will decrease as well, but I do want the Draka to have some significance, which is why I'll keep Von Shrakenberg's cameo, though his story will be different. I'll also add and remove some things, and create some numbers for immigrants.
Thanks for the feedback and help!