I think, if HRT isn't balanced properly (which I know is a controversial thing to say...) it can cause weight gain.
Basically, make sure you are taking enough progesterone. P is a diuretic and prevents the fluid retention. If you are just taking 100mg of it, that's quite a low dose and often isn't enough.
And then T, helps metabolic flexibility and muscle mass (with muscle being a caloric organ, burning energy) - and T often is not prescribed on the NHS.
Finally, my experience is that if you UNDER-eat or eat erratically, you gain weight. I know this sounds insane, but I was eating about 500-1200 calories a day and still either maintaining or even gaining weight. I used to think that it defied the laws of physics and energy or something. It was only when I started eating 3 meals a day and doing a reverse diet to gradually increase what I was eating, that I started to LOSE weight. Yes, I ate more and lost weight (I lost fat and gained muscle).
What has been key, is eating enough protein. Approx 30g for each meal and at least 1g per lb of body weight. (For me that's 144g of protein a day, but often I hit 150 or 160g of it.)
Basically, you need to look at the whole system and make it into a metabolic machine - rather than hyper-focussing on one thing (like a hormone or diet or exercise) - it requires a large scale systemic approach. Usually.
The two KEY most helpful resources for me, have been 1) a podcast called 'Metabolism and Menopause' (you can find it on Apple podcasts) - that helped most with nutrition and diet and how to do a reverse diet. And then 2) a structured strength workout programme I do at home with dumb bells. I love the Caroline Girvan app on the app store, which is called CGX. It gives you a whole programme to do so you can tick each one off as you do it. (Left to my own devices I would just not do them!).
Oh and also 3) get the app called My Fitness Pal so you can track what you eat and how much protein is in it. You don't always need to do this but it helps if you have no idea how many calories you're eating or how much protein. It was staggering how little protein I was eating before...
Hope some of that helps.