I've just read through some of your previous posts, and I'm truly sorry to learn about the rough patches you've been through.
It seems to me that your wife is trying very hard to (a) redeem herself for her previous lapses in judgement and (b) distinguish herself among her JW peers as someone who embraces The Truth™ completely to the point where other priorities take a back seat (you, your marriage, your children, working toward financial stability with you, your children's education, and so on). Understand that (b) is considered highly virtuous among JWs. It is also a sign (to me, at least) that she feels some unworthiness in herself among her JW peers - it's not uncommon for a person who has strayed from their marriage to Show Works Befitting Repentance™ (such as Pioneering™) to demonstrate that they've changed from their previous lapse in judgement.
As a teenager, I did some things during a Rebellious Phase™ that JWs weren't supposed to do, but once I started being ostracized by JWs (both at the KH and socially, including my mother), it triggered a high energy zeal for The Truth™ - wanting to go to all the Meetings™, isolating myself from Worldly™ peers, and wanting to get Baptized™ and Pioneer™. In retrospect, I can see the manipulation for what it was, and I was grateful that my application to Pioneer™ was rejected by the Elders™ at the time.
Given your circumstances, you could ring up an Elder™ and say that while you support her going to Meetings™ and in the Door-to-Door Ministry™, you need to draw the line on Pioneering™, as your children need her, and if she has that much spare time on her hands, you would appreciate it if she had a part-time job to help with the finances of the home. That's not to say you wouldn't reconsider once your youngest child is in school full days, but for now, you're asserting your Headship™ and asking her to prioritize the children's needs and the family's needs over Full-Time Service™. Tell her that you need to sit down with her and Count The Cost™ of Pioneering™ (none of this Jehovah Will Provide™ nonsense!!) - will she need her own vehicle? How will she pay for one? Surely she shouldn't expect you to take on the cost! Will she pay for the insurance and fuel and repairs? What about the child care arrangements - you have one that isn't yet old enough for school, so is she planning to take the baby with her or find child care for her? (Newsflash: JWs don't really like it when Sisters™ take their children along in Service™ - I tried more than a few times to do so, but I was basically shunned and not included in any car group - even when I offered to use our vehicle.) How do you feel about the children going Door-to-Door™ with her during the off-school times (weekends, school holidays, etc.)? Surely your feelings count for something!
If you deal with the Elders™ with this kind of approach, there's no way they would give her permission to Pioneer™. You have every right to assert yourself in this situation. Be careful not to overdo it, though, because the Elders™ could tell her that you are bad for her Spirituality™ or be the cause of Absolute Spiritual Endangerment™, which would give her the green light to leave you and take the children too. (Note that there is no Biblical basis for this belief.)