Putting up nesting boxes or not?

We're new to birding (and gardening, in fact) and have discovered many new birds in our garden since setting up a proper feeding station.

We have had robins nesting in a hole in the side of our house every year for the past three years and are immensely fond of them.

This autumn/winter, we now have sparrows, blue tits, coal tits, great tits and blackbirds feeding.

We would like to put up some nesting boxes, but have read some nasty things about sparrows attacking baby robins. They are certainly quite pushy over the feeders!

Should we just leave nature to it, or is putting up nesting boxes at different points of the garden reasonable? Our garden is approximately 50m long.

If we were going to try and cater for the blackbird specifically, are there any good tactics for this? We have read that blackbirds don't always have the best luck keeping their nests safe and would like to help if we can. There seems to be one in our garden almost permanently. Unlike the sparrows, it keeps itself to itself.

Thanks in advance.