Confused about how much I should be eating

So it basically sounds like you are doing a recomp at the moment. And you are correct, recomp is slow. If you want to gain muscle and weight, you will gain fat also, but it will take more food/calories than you are currently eating.

A half hour of cardio would not burn 600 calories, neither would 1.5 hr upper body strength. Added together... possibly. What selections are you using from the cardio database?

Just to unpick a few things....

"I’m experimenting with IF in hopes of burning off my last bit of belly fat and I think that’s why"
IF doesn't burn any more body fat than the same calories spread throughout the day. It can help some people (like me) budget their calorie allowance easier. What people fail to take into account with IF is that although you may burn more body fat during your fasting period that's balanced by storing more body fat during your eating window.
Whether you are doing IF or not you are burning both dietary and body fat 24x7 as part of being a living being - no forcing is required!

"I try to eat so I’m not in a deficit, the problem being figuring out how much that actually is."
Your weight trend over weeks and months is your best guide. Adjust, monitor for a month, adjust again until you find the sweetspot (and tracking methods) that help you stay in your desired weight range

"but for example, today I did 1.5 hours upper body strength training and .5 hour cardio, and both are saying I burned 611 calories"
This sentence can have many different meanings so can you clarify?
Are you saying an exercise only estimate for 30mins of cardio and 1.5hrs of weights both came to the same number?
But are you sure this is an exercise only estimate or is it a Fitbit adjustment that takes all your movement, activity and exercise so far that day into account?
Or are you saying both MFP and Fitbit reported the same number? (Which they would if they were synched.)

" but in general my weight hasn’t really changed since November."
Which means you have found your weight maintenance calories. But only for that time period - if your activity and/or exercise changes your maintenance calories change too.
Were you using your Fitbit for your calorie goal or MFP?
Losing 1lb in a couple of weeks is both too small a number and too short a timescale to mean anything. I'm up 3lb in 24 hours, which obviously doesn't mean I ate 4,500cals over maintenance yesterday!

"Tl;dr: how do I really know I’m eating enough? Do I just need to experiment and risk gaining fat? Am I sabotaging my gains by erring on the side of caution and risking being in a caloric deficit?"
By your weight trend.
Yes you should experiment - far too many people have an irrational fear of experimenting and choose to live with their diet, activity and exercise constantly suppressed when they could eat more and still maintain but with more food, more energy and better exercise perforance.
Think how long a 100cal/day true surplus would take to gain just 1lb of fat.... Which of course is totally and easily reversed.

As for recomp or bulk/cut choices.
Very, very few people NEED bulk/cut cycles to progress. Some people that don't need to simply prefer that approach though.
Just as some people prefer to avoid dieting to lose weight and eating in a surplus and the precision needed to do both those things well.
I would say as a generalisation added focus on most people's quality and quantity of training is of far greater significance than how they manage their weight a few hundred calories either way. Your goals and preference determine your choices,


Just for clarity It's not clear:
If you are using Fitbit or MFP to set your calorie goal.
How you are tracking / estimating exercise.
Your physique or exercise goals.

Is your Fitbit and mfp connected?

If they are connected please don't input exercise manually on MFP.

When you do so, a working setup overrides what Fitbit detected with your manual input.

Let Fitbit automatically detect your exercise and calories or correct an eggregious issue directly on Fitbit.

The exercise adjustment you see on MFP is just called that. It corresponds to your whole day calories adjustment and not a specific exercise. It will be finalized at midnight, so watch for that if you tend to go to bed early.

Your weight trend over sufficient time (usually at least one monthly cycle, if not two) will tell you if you're generally gaining, losing, or maintaining. The mirror and your performance may provide some insight into what!!!

However, other than water weight retention at the start of new exercise, weight gains should be extremely modest if they are primarily lean mass. I would argue in the lb or so a month range.