I would recommend seeing a financial adviser. I reckon your partner is early/mid 50s. This means she has yet to accumulate enough NI credits towards a state pension (she would have some, but not enough, credits if claiming child benefit while the children were under 18). You’ll not only need to discuss financial projections for retirement, but also likely upcoming costs, which can be significant. House deposits and wedding costs for the children? What if all the windows in the house need replacing? What if you become very ill?
If she’s early 50s, possibly menopause is kicking in and if the parenting was hard going, then that’s a double whammy of tiredness. Maybe a compromise? 6 months of carefree catching up with friends, not constantly cooking and cleaning, some new hobbies - like a sabbatical, a well earned one.
Generally, I think most SAHPs go back to full time work when children are at secondary school. Your partner is unusual. Most people have to put pedal to the metal on trying to pay the mortgage off. In your situation, there is the possibility that an additional wage will help you both to retire early.
Has she said what she will do with all her time? Many people who retire early get utterly bored. Complete retirement in the old fashioned sense (sitting down and putting feet up) is also associated with significant cognitive and physical decline. Does your partner realise this?
Really, you both just need to talk about this. She’s assuming you will be bankrolling her lifestyle. That’s not fair. It may be that she’s predicting a tsunami of elderly parent caring responsibilities, or wants to do future childcare for grandchildren, so sees this time as an opportunity to catch breath.
My husband faced exactly this situation with his ex. She was a spender. When the children went to secondary school, he asked her to return to full time work. She refused, thinking that she’d somehow earnt a retirement. She sat at home drinking prosecco and reading trashy magazines. The tax bills mounted and the mortgage interest rate went up and up. He was beside himself with stress, at an age where you can’t bounce back from redundancy (a big risk for men in their 50s). Needless to say, the situation ended in divorce and I met an amazing, kind man. We both work full time and we’re in a proper partnership, where risk and reward is equally shared.