So to be clear - your estimated TDEE by your best guess using Scooby and 5 level TDEE table is 2285.

Your TDEE is NOT 2285.

Your best estimated TDEE by your Fitbit is somewhere in 2300-2800, you mention high and lows, but not average.

That is also NOT your TDEE.

They are both estimates - one better than the other, especially if corrected for exercise it's not good at calculating calorie burn on. Like weight lifting - you manually log that on Fitbit, right?

But even the Fitbit has several reasons why it is underestimating.

If your potential TDEE is really up that high - you eating 1800-2200 should cause weight loss.

If your suppressed TDEE is not up that high - you could indeed be eating at maintenance.

Exercise does NOT cause weight loss - side effect is actually usually water weight gain if anything.
But it can transform the body so as fat is lost - there is something there you like.
But your body decides where fat is lost. Should show up somewhere if weight is lost.
And if getting good workouts in, should show up even if maintaining weight.

But workouts are subpar and body doesn't transform as much if you have suppressed system.
Why make improvements that require more energy when it's had to slow down already from not having what it wants?

Are you undereating - depends on if your body ever sped all the way up to potential TDEE or not.

But at least according to Fitbit - you selected the wrong TDEE level - you are between or higher - but why not use the Fitbit for what it's best at - infinite daily levels - you just need an average.

Very slow weight gain and waist increasing sound like eating over potential TDEE (but then workout performance should be increasing great) - unless it can be explained by regular binge moments eating over a suppressed TDEE and gaining fat, but then normally eating at suppressed TDEE.

Are you lifting?
If you are, are you increasing in performance nicely?
Or if not, do you have regular instances of eating above your calorie goal by decent amount?

Your suppressed TDEE is what you eat and don't change weight with.

So 2 directions - you'll have to pick.

Keep eating less - your body can only suppress so much, around 20% in studies. So if Fitbit estimate for healthy body is 2460, that means suppressed 20% is only burning 1968.

So that would explain why eating 2000-2200 is maintaining and gaining fat, depending on which side of range.

Now - with suppressed system, stress is increased, constantly elevated cortisol causes water retention, and even if you ate say 1750 to have a real 250 cal deficit or 1 lb every 2 weeks - your water weight gain could easy mask if not cause scale to go up, even as you lost fat.

With suppressed system - workouts won't be as good, takes longer to recover, so need less workouts perhaps. And body will be even less likely to make improvements since it's trying to conserve energy.

If eating 1750 now, how low will it be to actually reach goal weight, or how much more exercise must be done just to increase calorie burn.

How low will maintenance be, and can you adhere for a year or longer to lower eating levels until body has sped back up?

Or,

Realize that water weight gained while body speeds up is NOT getting fatter, it's getting heavier. That's not the same thing obviously.

You eat more than suppressed TDEE by small amounts to speed it back up to potential TDEE. Not infrequently enough the body stays slow and merely adds on extra calories as fat.
But every day so body knows it can speed up and unstress.

You'll likely find the workouts get better and stronger, which means the body can start asking for transforming things without this too big an energy deficit being in place scaring it.

There is no way to test your TDEE to see if suppressed or not - the side effects show it up.
You'd need research study to measure your RMR and bodyfat%, to see if RMR is lower than expected.
Then a normal day of activity after taking double-labeled water to see what the daily burn is.

Otherwise it's a matter of noting the effects, and testing how high you can go. Body is always willing to slow down if needed.

Correct, 100 more daily for a week, or two, at a time increases.

Your suppressed TDEE is likely what you are eating right now on average, probably the low end. The days of going high just add fat.

Which indeed sounds like about 2000.

Keep workouts going - should notice increase of performance as recovery is better eating more.

Fat is not fast, gain or loss.
Fast is water, gain or loss.

If you ate 100 more than real potential TDEE, it would take 35 days to slowly gain 1 lb, and if lifting not even fat.
250 more daily would take 2 weeks for 1 lb slowly.
Reread that if you need assurance.

As to your question of how do you know you've gone high enough - do you maintain weight?

Well, you are maintaining right now actually, right, but does this appear to be potential TDEE?
No, and you'll prove it out over the next few weeks by increasing calories and likely not increasing weight much at all.

How do you finally test it out, 2 weeks at 250 more daily - should be just 1 lb more slowly.

During this time of increase of a mere 100 calories - reflect on how slow fat gain would be.
If you are majorily under potential TDEE, body has adapted by storing less glucose with attached water in your muscles. That is extra LBM and extra metabolism burn, so it dropped it to conserve calories.
So you may gain fast 1-2 lbs of water weight and LBM and increased metabolism, but that maxes out. Though that increase could take couple weeks.