I started to write out all of the ways that she needed coping mechanisms for this difficult time (2 little kids) and therapy could help. But you know what? Therapy will only help if she has an open mind and willingness to delve into her issues. She has given herself A LOT OF COPING MECHANISMS already. She is not a FT stay at home parent. She does it PT. (As a FT working mom of a 4.5 year old and 6 month old, I was getting ready to saddle my high horse and get all judgy. I will try not to do that.) As a lot of other commenters have said, half day daycare 6 days a week in non-wealthy circles for a SAHM is not the norm. It is not normal at all. Given the ages of your kids, they both are most likely also napping/having quiet time in the afternoon when she is home and in charge of them. Which leaves her single parenting probably about 5 hours a day total, all added together. That's still work, but its not full time and it isn't what most people understand that they have signed up for in exchange for getting the privilege of getting to parent without the grueling FT parenting+workday drill. But if you acknowledge that she gets a lot of extra help each day and it works for you all, great.
Your wife may be a lovely person, but she is not coming across as a capable, strong person. You love her, say she is not lazy, and if you have the financial means to prop her up with lots of extra help (but not to do that and save for FIRE as much as you would like), I think you should work on accepting the situation for now, leave the lines of communication open when discussing pros and cons of saving more vs. spending, and see if things don't change when the kids get older and go to school in a few years (fingers crossed, for your budgeting purposes, public school). If she genuinely wants to save, but is just a fragile person that needs extra assistance during these 'tough' years, perhaps she will come around when times aren't as tough. But then again, I am relying on what I hear from others that school aged kids are, as a whole, much less demanding than toddlers/preschoolers. Good luck.