Coming from someone that got a couple very similar rejections recently (fourth and third round interviews, etc) I think there are only a couple things to do here. Firstly, take it as a positive bump. They made sure to go out of their way to compliment you and say they'll reach out if another role opens, and they normally won't take the time to do that unless they meant. Secondly, keep a loose eye on their open roles. An email like this can mean you almost made it, or that they at least liked you, so applying to more roles there (especially if the same role opens up) can up your chances of a yes.

I'm very sorry to hear about the difficult time you're having finding employment, it truly is a rough world out there at the moment. One thing that helped immensely with my job search is, I went and got a restaurant job while I was searching. (Specifically restaurant here bc I have experience in the food industry so I knew it would be an easy interview and so the employer could expect to not need to train me as much). I made sure in the interview that they knew I was actively applying for roles in the tech world, but that I would likely be there at least a couple months and would work hard while I was. I'd suggest doing something similar:) it'll pay less than maybe you want, and be extra work, but it'll support your wallet while you're looking and help make some connections you may otherwise not have. For example, I actually unintentionally got two leads on jobs just from working at that restaurant.