If you're already living together, then whether you're legally married should not be an overarching concern. Rather than formally propose, you should simply mention the idea in passing and see how much interest is shown. If not, then wait a year.
If not living together, then address that concern first. If not ready to live together, then not ready to marry.
Once two people are living together, separating is going to be painful whether you're legally married or not.
Sorry if I don't seem very romantic. To me, marriage seems like something two people will do when they both want to and are ready. Having said that, it genuinely perplexes me why so many couples live together for so many years without getting married, given that they are already married for all intents and purposes, except for the paperwork. I guess it has to do with the fact that people still think they have to spend thousands on an elaborate wedding, reception, etc. along with bachelor/bachelorette parties, bridal showers, etc. Only reason I can think of.