In the sparse drier regions back then, the population seem to be distributed around the Rivers where Agriculture was possible without much use of Technology like building large canals or dams. The regions of Rajasthan, Sindh, Madhya Pradesh, Gujarat are very dry but Snow fed rivers aren't far away and building canals would make building European colonial Settler colonies possible.
If empty regions of Afghanistan are annexed, it would give much more empty land with good supply of Snow fed rivers there too. I'm unsure about Persia but there's a lot of empty land with rivers there too.
I'm not sure if you get it but India was densely populated just about everywhere, you need first to ask why would Europeans go an create inland ethnic enclaves in India which would not only require to deal with a very unfamiliar climate but also have to drive millions of locals off in the hope of gathering enough settlers to create any hope of having even a local majority.
The areas that are uninhabited are for a good reason, you can't just expect them to move river water around and tap every single "empty space". They didn't exactly manage in Rhodesia or even South Africa or Namibia why would they in mainland Asia?
I would rather look at areas without many people to begin with and that are actually habitable(as opposed to Europeans somehow becoming "bedouins" and living in formerly desolated area between native settlements lol) like Manchuria, pre-Meiji Hokkaido, Central Asia(like OTL Soviet colonization but earlier), maybe even Dzungaria. That's the Northern Asian front, in Oceania and the Indian ocean you could maybe have European or mixed European-African/Indian islands if the Brits decide not to use Indian forced labour and on top of that encourage Europeans to move(not sure if there is an enough of an incentive on both sides but at least it can feasibly happen demographically).
Maybe you could have more Europeans in Singapore, Malaysia but really with a post-1800 POD I don't think you can really expect that much more colonization when OTL colonies and new world nations still are around to pull people towards them.