I am by no means an expert but below is my opinion and if I am telling you something you already know then I apologise in advance.
Firstly I commend you on taking steps to try and help yourself, It isn't easy to find motivation when dealing with mental health conditions, so really well done for putting yourself out there and sharing your story.
Mental health problems teamed with stress can lead to binge eating as a coping mechanism, I say this from experience.
I too have been down lighter life route many years ago, and it took doing that type of diet to realise starvation was not the answer. Also that I realised a diet isn't something you go on, a diet is what you eat.
You mention that you have been using Huel for a while already, and I think your challenge to do 100% Huel for a month is a great idea, however unless you are already having Huel daily to cover 60-70% of your calorific requirements, I personally think going 100% Huel without building up to it could be an added stress.
If you do decide to try 100% Huel for a month then I think it would help to have already experimented with your recepies if you are going to be adding a bit of fruit or coffee etc, just so that you can enjoy your shakes.
Finally picking up on your comments regarding calorie aims, my personal advice to you would be three things:-
- Use a calculator such as https://tdeecalculator.net/
As an example I have used it filling age as 30 years old, and the calculation shows that if you consume approx 2800 calories a day your weight will not change.
Therefore a good starting point would be 500 calories less, 2300 calories a day.
From there on out you could use the calculator every couple of weeks and gradually lower your calorie intake keeping the deficit the same and over the course of time if your activity levels rise then it is a really handy tool to ensure you consistently losing weight.
If the calculator tells you 2300 cals a day is your first target but you know yourself that 2000 calories is perfectly satisfying already then start there, listen to your body.
Keep a food diary or use myfitnesspal, super helpful even if you end up on Huel alone it keeps you accountable and gives you one place to log your food and progress on your weight loss, you can store body measurments and even progress photos and even activity levels.
As I said above this is purely my opinion and I have taken the time to write this info for you as I can relate to your post and admire that you have taken the first step of making a plan.