@crosswise I've been through similar, and I know you will staunchly get replies of "CICO" and "you're obviously doing it wrong", it's very frustrating!
I found Herman Pontzer's book "burn" quite enlightening. The gist of it is, yes CICO matters, but your CICO needs can adapt, and your body will always figure out a way to survive on the calories you give it. I think this is what people refer to as "starvation mode", but it's really just metabolic adaptation. It is strange that is started right away for you though, not after a period of time.
The next thing you might want to look into is caloric availability. Your body cannot extract all the calories from the food you eat. Think of sweetcorn - you might eat 100 calories of the stuff, but you definitely know a proportion of that isn't digested, as the evidence is visible. Consider how you could adjust the foods you eat to suit. 100 calories of nuts and 100 calories of chocolate are not the same.
The one final thing that can be worth considering is underlying issues, like gut health, nutrient deficiencies etc. I struggled with weight loss on a very restrictive diet, but discovered vitamin D deficiency and thyroid antibodies. When I started trying to address them, the weight started shifting on the same diet as previously.