I have reduced my alcohol intake, lost weight and have been taken medication that isn't an immunosuppressant (acetretin), for the psoriasis, along with the NHS giving me a free flu jab as I'm in an at-risk group despite being only in my early 40s.
I was taking vitamin D back in 2017 after it was recommended to me by one of the doctors, but slowly got out of the habit, and I'm now wondering if it can help with some of this. I have a jar of 3000 UI knocking around, so I'll start taking it today, and see how this goes - I have an outward looking signal on how well my immune system is behaving based just on my skin condition, which I've lived with for years so should be a good indicator of whether it improves or not.
If you want to know more, just ping me via the channels mentioned in my profile and I'll happily update in a month or two - let's see if I can provide a sense of it!