You need to include the version bounds if you want auto-merging to happen. If you don’t, someone will need to merge your new version by hand (and I guess you need to explain to them why you leave them away ;-).
And, yes, like this. Of course you should check that the version you use now works with the dependent packages at this point, ideally, by running your comprehensive test-suite…
I wouldn’t bother to find out what older versions also work, as that is very tedious. If someone needs something specific they can open an issue