OP much like you I found that Vyvanse was variable and often caused a lot of residual stimulation that made it hard to sleep even at only 30mg a day.
Because lisdex has to be metabolised into dex by your body removing the lysine molecule, how strong the drug feels depends very much on your metabolism on any given day.
I have found this still happens to some extent with dex, likely for the reason
@CFC gives above, but it's far less severe. Some days 10mg will feel stronger than it did yesterday but, just to pull some numbers out my arse for a minute, the variation is more like 20% whereas on lisdex it was 80%.
So I never had any idea how lisdex was going to effect me on any given day and it's not like you can titrate the dose throughout the day because it takes two hours to kick in and lasts up to 12 hours... now with dex if I don't use it for a few days and am unsure how strong it'll feel, I can just redose as needed, or break a 10mg tablet in half, and so on. It's much more flexible and predicable and I find it to be superior medication for this reason.
That said being conscious of your history with stims, I do hope you can resist the urge to abuse them, because it sounds like they're doing you a lot of good medically.
As for others who've said the doc should have started him on IR, that's just not how it works in the UK. The medical guidelines in the UK state to try XR stims first as they're easier for patients (one dose a day) and are less prone to abuse. They like lisdex in particular because you cannot just bypass the time release by crushing it up, since it's a slow release prodrug you'd need to do some actual chemistry to bypass it.
I went through pretty much the same path as OP from another private specialist. I got Concerta (time release Ritalin) first, then Vyvanse, then dex. The dex is the best med in my opinion.
You cannot really get Adderall in the UK. Technically you actually can privately but it costs far more than it's worth and dex is cleaner anyway.