We could use [space], option+[space], and option+shift+[space] to give a regular space, an en space, and an em space respectively. This would be similar to hyphen-minus/en-dash/em-dash, so easy to remember. And it could be done in software.

I was going to say you need 5 space buttons to chord all possible spaces, but I guess you can use 3 space buttons in combination with ctrl, (alt|option), and (win|command).

You are probably joking, but there are typewriters with split space bars exactly for this reason.

Why three? Only two kinds of space are necessary for the vast majority of English-language text: one to delimit words, and one to delimit sentences.