Is buying iPhone 6s Plus in 2019 a sensible decision
Depends on what price you are getting it at.
Then I don't think it's a good choice as you can get some good android options at this price range.
I am having an iPad and I am finding it difficult to transfer files between my redmi and iPad. So I thought of buying an iPhone which is affordable for me .
No, it is not a good idea. Only buy the phones that Apple is officially selling, which you can check here. At least go for the iPhone 7. Also, it is a good idea to plan ahead and buy around October-December, when prices of the older models have dropped, and sales are going on.
nope coming from apple ecosystem i personally wont recommend apple now they are too greedy now