I have a poached egg and some brocolli for breakfast (c120cals). Fills me up until lunchtime when I have 2 scoops of Huel (c400cals). Easy, easy to prepare and relatively cheap (as opposed to buying something on the day as too crap to make lunch otherwise).
My TDEE is 1500 (or probably less as am menopausal and underactive thyroid so struggle with weight).
So this leaves me 900 calories ish to eat dinner. More if I take into account a run done that morning (though I don't tend to think like that) And I eat anything. Fully butter/cheese loaded risotto, chicken and veg, pasta, pie, salads, mediterenean fish stews. Anything.
I tend to avoid bread Mon - Friday. I may have "proper" lunch at the weekend.
I maintain my 60kg weight with shoving in a few runs during the week.
If I veer off this I slowly put on a few pounds....but it goes again once I start back again,
I don't feel like I am in any way on a diet. The breakfast/lunch work well for me. Evening meal is diet free. Never hungry. Or Hangry.
So it is sustainable - if you can find something that works.
Oh and my 120 cal breakfast has been a revelation. Seriously keeps the hunger at bay - more than any porridge/cereal/bread based breakfast has ever done.