I think FitBit guidance is making me lose too much too fast

I agree with others: The main problem is your weight loss rate goal. It's too aggressive.

Your current very fast loss might or might not continue**, but with a goal of building/retaining muscle and performance, it's counterproductive to even try for 2 pounds a week. Having started on MFP at about your current weight (nearly 7 years ago, 154 pounds at 5'5"), I accidentally lost too fast (that fast) at first (because MFP underestimates my calorie needs by several hundred calories daily, as does my Garmin).

I got weak and fatigued, and it took several weeks to recover, even though I corrected quickly. I was lucky; worse things can happen, and all I experienced was that, and maybe bit more hair thinning than normal a few weeks later (that's usually a delayed effect, if caused by undereating).

** Usually, multi-pound scale drops or jumps over a few days, that aren't consistent with past patterns, are about changes in water retention or digestive contents on their way to become waste. They aren't about changes in body fat. Even if we don't know why something like water weight changed, it can do surprising things. (The body can be 60%+ water, after all.)

An aside about the calculators and trackers: All of those are calorie estimates. They spit out what amounts to an average number for someone of your characteristics, based on research. The estimate will be close for most people; a little off (under or over) for a few people; and quite far off for a rare few. That's the nature of statistical estimates. (Your Fitbit doesn't measure calories; it measures things like movement, heartbeats, etc., and estimates calories.)

You're not an average or a population, you're an individual. I you were calorie counting during the July-December loss, you can use those numbers to estimate your maintenance calories from your own data. Because you've changed weight over that time, you'll want to look at how much faster/slower you lost than expected based on TDEE/tracker estimated calories, or just use the last month or two of clean data (because you would've been closer to current weight then).

(Note: You probably know this, but we tend to burn fewer calories in daily life when thinner because it takes fewer calories to move through the world, with less body weight. Some people increase spontaneous movement when lighter, so not everyone sees that effect from bodyweight reduction, but I guarantee it's in the picture.)

To do that, add up all the calories you ate in some multi-week time period (ideally at least 4 weeks, or a whole menstrual cycle if adult/female/not menopausal). For the same time period, figure out your weight loss. Multiply the pounds lost by 3500 (rough number of calories in a pound of fat.) Add that number of calories to the total number of calories you ate in that time period. Divide the total by the number of days in the time period.

That's your approximate average TDEE to maintain weight, at the range of bodyweight during that time period, based on your own data. It's a better estimate than a calculator or device can give you, because it's more personalized.

Knock 250 calories off that to lose half a pound a week. Unlike some others, personally I think you might be OK losing a pound a week for another 10 pounds-ish, as long as you don't have lots of other physical/psychological stress in your life from other sources. (Oversimplifying, stress is cumulative across all sources, even positive sources like weight loss or energetic exercise, and it's stress that increases health risk, can cause other problems.) However, I agree that if your goals include muscle gain, half a pound a week is more compatible with that goal.

Best wishes!