Would I be wrong for accepting a promotion knowing I'm planning to leave?

I work part-time at a small company. I also do freelance work on the side in a related field. They've always known about the freelance stuff, and it's never been an issue.

Back in the winter I went to my boss and said I was hoping to go full-time with them if they could extend my role to forty hours. Instead, my hours were just increased and they said "let's see how it goes." No benefits, no pay increase, just the same work I'm doing with more hours to use a week. I later found out a coworker at the same hours already gets benefits, so that felt like I was being deceived a bit, but that's beside the point.

Because they didn't commit, I kept actively scaling my freelance business. That's only fair, right? I've got mouths to feed. Right now, the freelance part of my work is still a big part of my income.

Well now, months later, they're ready to have the full-time conversation. He says that they didn't know that I was still trying to grow my own freelance business since then, though, and that it felt like I was 'hiding' stuff from them (again, not hiding anything from anybody; I just can't afford to stop freelancing because part-time at a small company ain't going to pay anybody's bills, especially w/ no benefits). How they discovered I was still trying to grow my freelance work is another story.

Here's the part I feel conflicted about. In the time since I had that first convo about going full-time, I've started working toward a career change into a different/adjacent field. I'm actively studying for a certification and planning to sit for it in the fall. But I have no idea when I'll land my first gig in that field. Could be before EOY, could be next year, idk. The point being: there is no offer-in-hand in that field.

So the dilemma:

• If I turn down full-time, it's going to look suspicious given that I literally asked for it not that long ago. It'll probably create tension and raise questions I don't want to answer.
• If I accept it, I'm doing it knowing there's an expiration date. But I'd get better pay, a stronger resume line (which I don't super care about tbh), and stability while I finish my certification.

My thinking, right now, is to approach them and say "I'm happy to come full time on my terms" and name my price and name what I would want my full-time role to entail. If they agree, great. I'd continue working there for a few extra months. If they don't, then respectfully, I can say that my current arrangement will need to work for now.

My rule of thumb has always been to take every promotion/pay raise, regardless of anything else. Thoughts? WIBTA?