Hi apepper21
It sounds like you want to stay clear on your decision to keep on LC at work. I'm new to your conversation but I can offer these suggestions. LC is important to you and it's your boundary. Therefore the enforcement and monitoring will need to be done by you.
Therefore, I would handle this fear of yours--
Which maybe those ARE ok, but I fear they will lead to him slowly saying more and approaching me more and suddenly I'll be right back in it.
--similarly to how I would handle a basic boundary over time. I.e. you will be the one responsible to maintain it. You can start by writing it down on paper. It will help you remember it since it's important to you.
One way to look at your relationship with him is with two ropes. You have a working relationship with him and a romantic relationship with him. Each of these represents a horizontal rope that is attached to you. It sounds like you're "done" with the romantic relationship and you've committed to detach. Therefore, when he comes to you with a work-related issue, work with him using your working relationship rope. Keep it in your mind that you want to maintain a zero-interaction relationship with him on the "romantic relationship rope". Therefore things like (1) let's get a drink, or (2) come to my house are off-limits. The "gray" area ones like "let's get coffee/lunch/dinner" are best looked at seeing them as "gray" and making a conscious choice not to do it. You seem to know at this point that it's probably going to lead to trouble. And you've already been there.
Another way you can treat your work relationship with him is 'strictly as a colleague'. Envision him in your mind's eye as the guy you want to keep utterly in the friend zone.
I did well Wednesday an Thursday and I guess for the most part Friday, but I noticed myself on Friday (after being VERY angry in the morning) trying to make sure things were "ok" btw us. How did I do this? Well if we passed each other I'd smile, or say hi if he didn't. Kind of to get across, I can handle this maturely, don't need to hate each other and kind of to get across to him that I'm fine with it. If honest with myself though, ALSO to get a read on him I think... .
I think that's fine. It's consistent with being mature and strong enough to handle a working relationship. It signals a detached past romantic relationship. In fact I would think your behaviour is good because you want to signal healthy (healthy for you) things to him--so why the "but"?
Good luck!:)