Mother-in-law to be makes our relationship feel like I am in a relationship of 3

I (45 M) am engaged to my (45F) best friend. It has been an absolutely amazing relationship (3 years) and we are both truly excited about future. I can say unequivocally that I have found my partner and I am truly blessed.
In the very early beginning of our relationship (about 2 months), my SO needed to move (she was still in her martial home well past the divorce decree agreement) and was having trouble finding a home that was in her price range in the school district that she wanted to keep her kids (12M and 16F at the time) in.
Her mother (68F) was living alone and at the mother's suggestion and offer -- she sold her house and used the money as a downpayment on a house that was big enough for them to live together and for mutual advantage (my SO could stay in the town she wanted and her mother would no longer live alone).
They bought a beautiful 5 bedroom, 3000 sq foot home and while it doesnt have an in-laws apartment it does have 2 living rooms and a Master Bedroom that is separated from the other 4 bedrooms.
Fast forward, we are now engaged, I moved in about 3 months ago (I pay 1/3 of expenses) and so the household consists of me, my SO, her mother and my SO's youngest (15M) shared custody with her ex (so he is there M,W and every other weekend).
My challenge: From the Mother's POV: She moved in with her daughter for the purposes of not being alone and makes my SO feel guilty if she isnt involved in everything.
After work, when we come home, her mom involves herself into the "how was your day wind down" (complete with items to discuss etc).
Even though the mother doesnt cook, she expects to be included in all meals and have menus around her likes and dislikes.
If my SO and I go out (pre and post Covid) for dinner she pulls a guilt trip if she isnt included.
My SO feels the need to ask permission if we want to go away for the weekend or even if we are running errands we will get texts about dinner and needing to come home so she isnt alone "for too long".
When we do want to go away, we need to arrange for it to be a weekend she has her son or for my SO's daughter (20F) to come and spend the night with her so "Nana's" not alone.
Even when it comes to a night at home "alone", her mom makes my SO feel guilty if she is not included.
I would say that over the last 2 years, her mom has been included in 30% of our dates: dinners, shows, trips to the beach etc.
Her mom "hosts" holidays and cookouts etc, my SO and I cant visit my family together as my SO feels obligated to stay since it's also her house. I will also add that even though the mother is the "host", my SO and I do all the cooking and planning and guest related services although the mom cleans up.
We cant make any decisions without Nanas buy-in.
My SO is aware that her mom is too involved but doesnt know how to control it and she tries to play good cop to both sides, which actually causes her to be stressed. We joke that her mom is like another child but the reality is, it is interfering in our relationship. They get along, but are both strong willed and argue like a couple that she get a divorce on the grounds of irreconcilable differences.
My SO thinks that it's a short-term problem and that as soon as her son graduates from High School, we can separate but I dont thinl her mom sold her home of 20 years to then move again in 5 years -- we will have Nana for as long as she lives (which I hope is for a very long time).. I look it at that when they bought the house together, my SO solved a short term problem (staying in the school district) with a long term solution (living with her mom for the rest of her life)
I have no ill-will for her mom, we get along well. I just dont want to be in a relationship with her. I dont want friction but I need to have a Relationship that involves just my wife to be and I first. Living together and caring for her long term needs, no problem. If we are cooking at home and she wants to join, no problem. But I need to find a way to make it a relationship of the 2 of us. Maybe a better way of saying it.. I feel like my SO needs to put our relationship first.
TL/DNR: My SO's mom is too involved with our daily life and relationship and I need help with trying to separate the two and honestly put our relationship first on her priority list.


Source.