I’m not understanding MFP’s ‘goals’?

So 1200 calories was your base goal with being the Sedentary you likely selected.
And with no exercise being done.

But you are not sedentary.
And you do exercise.

So you should have never been eating 1200 calories if using MFP correctly.

The 1.5 lbs weekly was likely reasonable when you had over 30 lbs to healthy weight to lose.

As older woman the ease by which you could lose muscle mass with too aggressive a diet is very bad.

Suggest you please back off the deficit if really down to last 12-13 lbs to only 1/2 lb weekly, 250 defecit.

And if you have Garmin linked and syncing your daily eating goal would have gone up correctly as you did more.

If not linked and syncing, then get your activity level off Sedentary you are not, and 18K steps is probably enough distance you are Active.
With that set honestly, the 45 min x 3 balance/strength won't be as bad a hit if you don't log it.

How many calories burned daily does Garmin Connect say you average in say last 2-4 weeks?

So it's a common mistake to think the MFP activity levels have something to do with exercise - they do not at all - hence the reason exercise is added when actually done.

That level of steps outside of exercise would indicate if you did not sync with Garmin - you would honestly be selecting Very Active, yes, the top level for that much movement.
With it synced MFP is attempting to correct itself to Garmin, if you kept trusting MFP anyway as you seem to on one level.
That 18K steps was my 10 mile run on Sat.
How much distance are you getting?
And you don't think that counts?
Those extra calories in the Exercise Diary are NOT exercise - merely the difference between what MFP thought you'd burn by your incorrectly guessed activity level - and what Garmin reports you are burning. In your case little to none are exercise, but rather daily movement - and you burn extra calories doing that.

Garmin Active Calories is not calories burned daily.
You can't figure out how much deficit you in theory have by using Active calories.
Total Calories Burned is the figure.

Now with that many steps, calories based on the distance moved means accuracy of your stride length is much more important.

Be very concerned you are not hungry doing that much - that's a bad thing, means your body is likely adapting in all kinds of wrong ways to attempt to stop the craziness.

Calories is the figure you want. No need to do any math. Website calls it what I called it - was staring right at it when I typed it in.

You lose fat weight by eating less than you burn. That simple.

You need an idea of what you burn - in total. Not just the active calories, not just your BMR (which is Basal Metabolism, or sleep calorie burn rate), both those by themselves are not useful.

You need an idea of what you eat - in total. Not just for breakfast, not just for snacks.

If you are going to do some math - you'll need the correct numbers and understand them.
Great plan though if you have decently accurate food logging, probably 4 weeks worth of data minimum.

You will gain scale weight eating more - your body has probably been itching to store more glycogen in the muscles - that stores with water.
It's part of the big weight drop in 1st week people start a diet. And then it's regained when they eat more.
Suggest just 200 cal more daily for a week or two if that much to adjust by.

Now - you may have energy level go up in ways you've never noticed when it went lower, so your daily burn may go up beyond what any device on you could discern and estimate.

So you are getting some massive distance from those steps daily. Well beyond MFP Activity Levels.
Accurate or not would depend on testing on a known distance track, like walking a known 1/2 to 1 mile.
Start a workout at the start, end it when done - now you'll know steps for a known distance.
It should be at a middle daily pace - about 1.8 mph - which will seem slow. But that's the middle of your daily range probably.

Got some good plans there, very nice active lifestyle, and useful workouts it sounds like.
1 less big stress in life is always useful. Let's you add your own useful stress in metered amounts.

Be aware most HS tracks have been converted to metric - you'll have to find the marks for 1/4 mile to get 1/2 or 1 mile. 4 laps doesn't get it anymore on those tracks.

So you are ready to get roaring on some more serious workouts - nice.