Do you trust your wife? A few of your statements indicate that you don't really, and she may be trying to tell you that she feels stifled and suffocated by how tightly you control your finances. That isn't to bash on you, but it seems like you aren't really listening to her. The statement "you won't be watching every penny" sounds to me like she is not very happy with the current set-up and wants a bit of freedom.
It is far less costly to loosen the reigns slightly than to divorce. :) Taking into account your partners needs and wants makes for a healthy marriage. You say she is a frugal person, maybe not to your standards, but she isn't about to blow everything on a new hummer or something. She sounds like a reasonable human. She is not an opponent or adversary, she is on your team.
Is she on board with the goals that you mention? Did you arrive at these goals together with a conversation over wine and chocolate, or did you decide these and then impose them on her without much input?
Many marriages function with separate finances. Mine does not, everything is one big pot, but I know a few that do. I think your wife having a separate account for fun, judgement-free spending is a fantastic idea, as long as the amount that goes INTO this account is agreed upon by you. Otherwise, you could also have a line-item in the joint budget for judgement-free spending for each partner. This is what my DH and I currently do, we each get $100 per month. I personally usually save mine, but DH is a tech junky, so every so often a new tech toy appears in our house and I say zip, nada about it, because this is planned for and I trust him to not go over-budget (and he doesn't, he is a reasonable human). Judgement-free, remember? That's what keeps a marriage running smoothly.
Also, I am just confused. If she is bringing in $20k extra, how would that possibly throw anything up in the air unless she would suddenly be spending MORE than she is bringing in? Is that your fear, that she earns $20k but then feels entitled to spend $30k just on herself? On what, a new car every year? Anything she brings in would only be a bonus, and I can pretty well assure you, unless you married a giant spendy-pants, she wont inflate the household spending by an extra $20k per year or more.
Additionally, your current situation sounds exactly like where DH and I would like to be in 6 or so years. I want to leave the workforce to have kids, but then I'm interested in going back and doing something part-time. I like the feeling of contributing, I'll likely want to get out of the house and have a few kid-free hours per week, and I feel that having DH be the sole earner would be a negative for me psychologically. But we have a spending plan and shared vision/goals, so everything that we both bring in goes into one big pot and gets allocated based on what we BOTH agreed to. If I were 6 years from now and felt that I had very little say in our spending plan and also little power/income, and feeling kind of restricted or judged/watched like I was a child, I would be feeling a little resentful and eager to have some money that *I* had some say over. Thats the general impression that I'm getting from your post, but I could be entirely off-base.
I would encourage you to simply talk to your wife. See what she likes and dislikes about your current spending plan/budget, and if she had a magic wand, what would she change? What is she actually frustrated about? There could be a very simple solution that meets both of your needs, but first, you need to find out what those needs are in a kind, gentle way. Talk about what makes her feel suffocated or uncomfortable with the current system. Talk about what she wants to spend "her" money on, or what kind of budget she wants per month. It could be as little as $100 per month just so she can go to the spa sometimes or get her hair done, or maybe $500. There is no wrong answer here, as long as that budget is below what she earns. Good luck!