Will this policy advantage the financial sector over other parts of the economy?
I do not see how this would a priori advantage the financial sector over other parts of the economy. This policy is done to stimulate economic activity by putting pressure on lowering the long-term interest rates and to push pressure on inflation and lower exchange rate. Such expansionary monetary policy is conventionally considered to stimulate real economic activity in the short-run (see any undergraduate macro textbook for example Blanchard et al. Macroeconomics: a European Perspective). There is no reason to a priori think this specially advantages banks over other sectors, as this is macroeconomic policy with aim to stimulate the whole economy, although banks will likely benefit as well.
Why doesn't the reserve bank give this $100B to treasury directly, so that fewer bonds need be issued to support gov spending?
This is for several reasons. First, giving money directly to government would mean that money supply is permanently being increased by that new money (technically that money could be still destroyed in some ways but not unliterary just by central bank). When central bank buys government bonds the money supply is expanded only as long as the government bonds are not repaid. Central banks still have option of scrapping the bonds and hence making the money supply expansion permanent, but they also have other options. Central banks will usually want to expand money supply in recession but in expansions they might want to contract it. They could contract it also with other ways (like increasing reserve rations), but open market operations are usually one of the preferred tools of central banks.
However, this does not affect government budget negatively. Any interest that government pays to central bank is then returned to the government. So from the perspective of government this debt has no cost. In fact such extra demand on government bonds will likely make it cheaper for government to issue bonds on primary market so if anything it might indirectly help it.
Also, the reason why it is done on secondary market rather than primary market, is that central bank is supposed to be independent from government. Hence they try to always keep arms length relationship with it.
Alternatively, why doesn't the reserve bank distribute this $100B directly to ordinary households (e.g. \$4k per resident)?
This is what is called 'helicopter money'. Some economists are in favor of such policy, however arguments are being made that it threatens central bank independence. Helicopter money is very close to fiscal policy and if people learn they could get 'free money' from central bank they might put political pressures on central banks being involved in such transfers beyond what is warranted by monetary policy. It is argued that any fiscal policy should be done by elected representatives and not by unelected technocrats, and helicopter money is very fiscal-esque. See more on this in Reichlin; Turner; Woodford (23 September 2019). Central bank independence is in economic profession revered in similar way as in law profession independent judiciary or in journalism independent press so anything that threatens central bank independence is hard sell. Of course, this raises additional question of why the central bank independence is so important - exploring that is beyond the scope of this answer but you can read more about it in this ECB explainer. In addition in most countries this would not even be legal although laws can be changed.
Theory above aside, even when we look empirically at aggregate measures of bank profitability we can see that in the post 2007 they are low, as you can see from the figure below which plots price-to-book ratio for banks, which is a measure of profitability and expected profitability that stock prices capture (see Carletti, Claessens, Fatás, Vives 18 June 2020, and sources cited therein).
QE was not common before 2007 and became quite commonplace after, due to the Great Recession. Hence, there is not much evidence to suspect that banks are somehow 'getting rich' out of QE. Furthermore, of course the above is just raw data so it has to be interpreted with caution. One could argue that maybe there are some other factors that depress bank profitability inspite of all the QE.
However, when we look at some serious research it actually confirms that QE together with negative interest rates and other recent bank regulation erode bank profitability (e.g. see Florian 2018). Hence, if anything there is research and evidence supporting the idea that QE, combined with other policies central banks nowadays commonly pursue, makes banks less profitable not more.