Wow...so much to think about.
Can you please elaborate on the scoring system you are using because I don't know it and no where do I know of a animal scoring the higher side of numbers being "healthy" and great....middle of the road is normally where you find healthy.
Please give us a picture of your horse so we can see what it is you make comment on cause I'm perplexed in what scoring system you are using.
The only scoring system I am aware of and follow/use is the Henneke System industry wide used for scoring a horses physical condition seen.
Don Henneke, PhD, developed the Henneke Body Condition Scoring System during his graduate study at Texas A & M University . It is based on both visual appraisal and palpable fat cover of the six major points of the horse that are most responsive to...
www.habitatforhorses.org
There are hundreds of links to slightly different picture displays but the definition remains nearly identical.
Both of the below horses are 8 on the Henneke score and need to lose some pounds..
If your horse is truly a 8 out of 9 on the Henneke system or even a 8 out of 10 the animal is well headed toward obese...
The amount of food you are offering is obviously excessive when the horse literally eats but can not eat all and leaves much as waste.
The horse is being given food to eat literally non-stop with what is given in a stall situation then roll of more hay in the t/o...
To me, removing the blanket will not change the appetite cause the appetite sounds great when the animal is presented so much food and cleans up as much as it does..
As for the Sr. feed....you are using a 4 pound container which held coffee.

Horse feed weighs considerably more in volume that coffee does....
Have you ever weighed it to see the weight of that container when it is filled with feed?
Bet it weighs more than 4 pounds.... You need minimum amounts fed for the vit/mi ratios to be met but you are also providing excess amounts of forage = hay of round roll unknown pounds eaten a day and then several flakes of higher nutritional mixed of legume and grasses...the horse is consuming large amounts of calories..
What is the horse doing to burn off the calories? Does she ride? Does she drive? Does she have companions who play and run with for hours a day?
Do you see any muscle mass or just fat?
Is there a cresty looking neck from fat deposits along it?
Can you see and easily feel any of her bones? Specifically her ribs, shoulder or wither or is all buried in a thick{er} layer of fat?
I would consider a vit/min ration that is not so many calories dense and a smaller quantity eaten and needed fed daily.
So...blanketing is used to offer protection from the elements that are adverse to the animal thriving and being comfortable.
Because the animal is fat by what I know as scoring numbers does not mean it is warm either if it not move around much because it is overweight either.
If you want to offer weather protection of keeping dry then get a turnout sheet that has no insulating properties but offers a windbreak and less chance of soaked through coat that indeed chills a animal to the bone cold in intensity...
Don't take away a "coat" thinking it will increase a appetite that is already sounds to me maxed out in amount eaten daily.
Take away the coat when the weather is warm from the sun, dry and no biting winds. If the horse is accustomed to having a blanket...then it probably did not grow as thick a coat as it needs to give it the protection against winter nasties.
Other have started to post as I was typing so...
please, get us a picture of the animal and a reference of the scoring system you are using so all can offer the best opinion and advice.
Where I think you will hear differing is in taking away a blanket the animal has come to depend on for its comfort level...to remove it now....just not what I would do, but I would consider a turnout sheet for protection of the elements but not necessarily for warmth extra when temps are mild in daylight hours of sun shining strong.
🐴...
jmo...