You can take a tough stance without S.
At the same time, I don't think S is a death knell. Why does he think this? Is he scared you'll grow away from him? Are you willing to grow away from him? In my opinion, S is a great way to put all the options on the table - but ALL the options can also be scary. You could realize you don't want him. He could take up cheating again far more easily. Those are options in S just as much as R is an option in S. Do you want to S so you can consider all your options? Or do you want S just to get more alone time and space? If the latter is the case, then I think you can do that without living separately.
When I had D-day 2 (major TT), I didn't know if I wanted to R or D. I was actually leaning towards D. But I wanted a few months before I pulled that trigger because I wasn't going to jerk my kids around and change my mind. So, in that month, we developed a pretty clear plan whereby I would get a lot of alone time to clear my head and take the focus off the M and put it on me. I didn't engage with him. We didn't talk about the As that much. I did a lot of yoga, socialized more, saw my IC and hung out alone in the house. We even worked out a schedule for the kids where we traded off and limited time together for awhile. He slept in another room. I guess you would call it in-house separation. Have you considered that?
Have you considered backing off MC? 7 months after D-Day 2 and I'm just starting to consider MC. I really believe that one of the "tough stance" things to do is to tell WS to get their shit together (at least a baseline level of shit together) and don't entertain working on the M until you see some progress. Jumping into MC seems to be greenlighting R. If you're not ready to do that, then why MC? Someone who TTs needs serious help before joint work on R can happen. If you need to S to get your head screwed on straight...then I would also argue that you should probably focus on your own IC. MC just seems futile when both people are scrambling, as it sounds you are, and as I was for a few months after D-Day 2.
These are just some things you could consider to be "tough" and get space rather than S.