The unpopular thing worth saying plainly: the day job is funding this and it goes first.
Not for moral reasons, for practical ones. The salary is what makes the side thing survivable, lets you say no to bad customers, and means you are not pricing out of desperation. Damage it and you lose the conditions that make the experiment possible at all.
So the boundary that matters most is the one at work. Phone away, notifications off, no checking between meetings. If something genuinely cannot wait until the evening, that is a signal about the offer rather than a reason to answer it at your desk.
On the partner, since you mentioned the deal: have the conversation again now, not when it becomes an argument. Say what has happened, that you noticed, and what you are changing. The version where this goes badly is the one where the creep is never acknowledged and resentment builds on both sides.
And set a review point, three months out, look at whether it is earning enough to justify what it costs, and be genuinely willing to stop or shrink it. A side thing that is quietly eating the week and not earning is a decision that has never been made rather than one that was made and is being honoured.