Advice (maybe encouragement) for maintenance??

I want to start by mentioning that this community has been one of the most helpful tools I had found for weightloss, so thank you all in advance for even looking at my post ❤️

I'm having some anxiety about maintenance mode, as many do 😅 5'7" 30F ~140lbs lost around 115lbs total in the past 2 ish years. I kind of dove past my goal and hit 137.5lbs about 3 weeks ago but knew I wasn't going to stay there (IBS-C, low water retention, IFKYK) but have been planning to sit in a range of 138 ish to 141, maybe 142 for fluctuations.

The concern: first 2 weeks I stayed around 138-139, but now on week 3 I'm sticking around 140. I'm TERRIFIED about gaining again, and I have no idea how to judge if my numbers are making sense and I'm just wiggling with water, or if my budget is lower than I hoped 🥲 logically I should still be fine, the math should math. But maybe I'm not taking something into consideration? Could water weight really swing this wildly while my body gets used to it's new normal??

The diet: Diet was sitting around 1200-1300 daily, and for the past 3 weeks I gradually got up to 1400-1500. TDEE should be 1600 ish with no activity, which is what I want to get to despite going to the gym near daily as I'm mostly sedentary outside of that. I track just about everything faithfully, and try to give myself a 50-100 cal daily buffer when having to do best guesses (like an unlabeled whipped cream, nothing big meal is generally guessed on)

If you got this far, I appreciate you more than I can say! 💞 Feel free to question, be kind, harsh, direct etc! I can take it 💪